LT3922-1
36V, 2.3A Synchronous Step-Up
LED Driver with 25,000:1 PWM Dimming
FEATURESDESCRIPTION±2.5% LED Current Regulation
n ±2% Output Voltage Regulationn 25,000:1 PWM Dimming at 100Hzn 128:1 Internal PWM Dimmingn Spread Spectrum Frequency Modulationn Silent Switcher® Architecture for Low EMI
n Operates in Boost, Buck Mode and Buck-Boost Moden 2.8V to 36V Input Voltage Rangen Up to 34V LED String Voltage
n 2.3A, 40V Internal Switches
n 200kHz to 2MHz Switching Frequency with SYNC
n Analog or Duty Cycle LED Current Control
n Open/Short LED Protection and Fault Indicationn Thermally Enhanced 28-Lead (4mm × 5mm) QFN
n AEC-Q100 Qualified for Automotive Applications

The LT®3922-1 is a monolithic, synchronous, step-up DC/
DC converter that utilizes fixed-frequency, peak current
control and provides PWM dimming for a string of LEDs.
The LED current is programmed by an analog voltage or
the duty cycle of pulses at the CTRL pin. The LT3922-1
will maintain ±2.5% current regulation through an externalsense resistor over a wide range of output voltages.

n

The switching frequency is programmable from 200kHz
to 2MHz by an external resistor at the RT pin or by an
external clock applied at the SYNC/SPRD pin. With theoptional spread spectrum frequency modulation enabled,
the frequency varies from 100% to 125% to reduce EMI.
The LT3922-1 also includes a driver for an external highside PMOS for PWM dimming and an internal PWM signal
generator for analog control of PWM dimming. When an
external signal is available, the LT3922-1 can perform
25,000:1 PWM dimming with 100Hz PWM pulses.

APPLICATIONS

Additional features include an accurate external reference
voltage for use with the CTRL and PWM pins, an LED
current monitor, an accurate EN/UVLO pin threshold,open-drain fault reporting for open-circuit and short-circuit
load conditions, and thermal shutdown.

Automotive and Industrial Lightingn Machine Visionn

All registered trademarks and trademarks are the property of their respective owners. Protectedby U.S. patents, including 7199560, 7321203, and other patents pending.

TYPICAL APPLICATION
30V, 333mA Boost LED Driver with 25,000:1 PWM Dimmming
L1, 2.2µH
VIN
8V TO 27V

SW

VIN
4.7µF

33µF

1M

BST

VOUT

0.47µF

GND

EN/UVLO

0.47µF

365k

VOUT
OVLO

LT3922-1

59.0k

ILED
100mA/DIV

1M
10µF

FB
33.2k

1µF

VREF

GND

CTRL

ISP

PWM

300mΩ

SYNC/SPRDINTVCC

ISN

100k

PWMTG
FAULT
SS RT

2.2µF
10nF
L1: WURTH 74437324022
M1: VISHAY Si2319CDS

VPWM
2V/DIV

RP
45.3k
2MHz

ISMON
VC
51k

M1

INFINITE PERSISTENCE
VIN = 12V
fPWM = 100Hz

200ns/DIV
39221 TA01b

30V
333mA
LED

1nF
39221 TA01a

Rev. A

Document Feedback

For more information www.analog.com

1

LT3922-1
ABSOLUTE MAXIMUM RATINGSPIN CONFIGURATION(Note 1)

VIN and EN/UVLO.......................................................40V
ISP, ISN, and VOUT.....................................................40V
ISP – ISN..................................................................0.3V
CTRL and FB.............................................................3.3V
OVLO, PWM, SYNC/SPRD, and FAULT.........................6V
SS and VC.................................................................3.3V
SW, BST, INTVCC, VREF, ISMON, PWMTG, RT, and RP (Note 2)
Operating Junction Temperature Range (Notes 3, 4)
LT3922E-1/LT3922I-1............................. –40 to 125°C
LT3922H-1.............................................. –40 to 150°C
Storage Temperature Range.......................–60 to 150°C

GND

VOUT

NC

NC

SW

SW

TOP VIEW

28 27 26 25 24 23
SW 1

22 GND

BST 2

21 VOUTINTVCC 3

20 PWMTG

VIN 4

19 PWM

29
GND

EN/UVLO 5

18 RP

OVLO 617 SYNC/SPRD

VREF 7

16 RT

CTRL 815 FAULT
ISMON

SS

FB

VC

ISP

ISN

9 10 11 12 13 14

UFD PACKAGE
28-LEAD (4mm × 5mm) PLASTIC QFNθJA = 25°C/W (AS MEASURED ON DEMO BOARD DC2247A), θJC = 3.4°C/W
EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCBORDER INFORMATIONLEAD FREE FINISH

TAPE AND REELPART MARKING*PACKAGE DESCRIPTIONTEMPERATURE RANGE

LT3922EUFD-1#PBF

LT3922EUFD-1#TRPBF

39221

28-Lead (4mm × 5mm) Plastic QFN–40°C to 125°C

LT3922IUFD-1#PBF

LT3922IUFD-1#TRPBF

39221

28-Lead (4mm × 5mm) Plastic QFN–40°C to 125°C

LT3922HUFD-1#PBF

LT3922HUFD-1#TRPBF

39221

28-Lead (4mm × 5mm) Plastic QFN–40°C to 150°C

LT3922EUFD-1#WPBF

LT3922EUFD-1#WTRPBF

39221

28-Lead (4mm × 5mm) Plastic QFN–40°C to 125°C

LT3922IUFD-1#WPBF

LT3922IUFD-1#WTRPBF

39221

28-Lead (4mm × 5mm) Plastic QFN–40°C to 125°C

LT3922HUFD-1#WPBF

LT3922HUFD-1#WTRPBF 39221

28-Lead (4mm × 5mm) Plastic QFN–40°C to 150°C

AUTOMOTIVE PRODUCTS**

Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
**Versions of this part are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. Thesemodels are designated with a #W suffix. Only the automotive grade products shown are available for use in automotive applications. Contact your local
Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for these models.

Rev. A2

For more information www.analog.com

LT3922-1
ELECTRICAL CHARACTERISTICS

The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 12V, VEN/UVLO = 2V unless otherwise noted.
PARAMETERCONDITIONSMINInput Voltage Range
VIN Pin Quiescent Current

VEN/UVLO = 1.5V, Not Switching
VEN/UVLO = 0.1V, Shutdown

EN/UVLO Threshold (Falling)

1.260

EN/UVLO Rising Hysteresis
EN/UVLO Pin Current

VEN/UVLO = 1.2V

Input OVLO Threshold (Rising)

1.145

Input OVLO Falling Hysteresis
OVLO Pin Current

TYP

MAX

UNITS

36

V

2.9

4
1

mA
µA

1.330

1.400

V

2.8

25

mV

2

µA

1.205

1.265

50
VOVLO = 1.0V

–100

V
mV

100

nA

2.03
2.015

V
V

Reference
VREF Voltage

IVREF = 10µA
IVREF = 500µA

VREF Pin Current Limit

VREF = 0V, Current Out of Pin

l

1.97
1.985

2
2
3.2

mA

LED Current Regulation
CTRL-Off Threshold (Falling)l200

CTRL-Off Rising Hysteresis

210

220

15
−100

mV
mV

CTRL Pin Current

VCTRL = 2V

Sense Voltage (VISP−VISN)(Analog Input)

VCTRL = 2V (100%), VISP = 24V
VCTRL = 0.75V (50%), VISP = 24V
VCTRL = 0.3V (5%), VISP = 24V

ISP Pin Current

VISP = 24.1V, VISN = 24V, VCTRL = 2V

ISN Pin Current

VISP = 24.1V, VISN = 24V , VCTRL = 2V

75

µA

Current Error Amplifier Transconductance

VISP = 24V

140

µA/V

l
l
l

97.5
48
3.5

100
50
5

100

nA

102.5
52
6.5

mV
mV
mV

75

µA

Duty Cycle Control of LED Current
Sense Voltage (VISP−VISN)(Duty Cycle Input)

CTRL Duty = 75% (100%), VISP = 24V
CTRL Duty = 37.5% (50%), VISP = 24V
CTRL Duty = 15% (5%), VISP = 24V

99
49
4

CTRL Pulse Input High (VIH)

100
50
5

101
51
6

1.6

V

CTRL Pulse Input Low (VIL)
CTRL Pulse Input Frequency Range

mV
mV
mV

10

0.4

V

200

kHz

Voltage Regulation
FB Regulation Voltage

VCTRL = 2V

FB Pin Current

FB in Regulation

l

1.175

1.200

−100

Voltage Error Amplifier Transconductance

1.225

V

100

nA

1000

µA/V

INTVCC Regulator
INTVCC Voltage
INTVCC Pin Current Limit

2.7
VINTVCC = 0V, Current Out of Pin

3
20

3.3

V
mA

Rev. AFor more information www.analog.com

3

LT3922-1
ELECTRICAL CHARACTERISTICS

The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C, VIN = 12V, VEN/UVLO = 2V unless otherwise noted.
PARAMETERCONDITIONS

MIN

TYP

MAX

UNITS

2.3

2.55

2.8

A

25

35

Power Stage
Peak Current Limit

VIN = 3V

Bottom Switch Minimum Off-Time

l

15

ns

Bottom Switch On-Resistance

140

mΩ

Top Switch On-Resistance

155

mΩ

Oscillator
Programmed Switching Frequency (fSW)

RT = 45.3k, VSYNC/SPRD = 0V
RT = 499k, VSYNC/SPRD = 0V

Spread Spectrum Frequency Range

RT = 45.3k, VSYNC/SPRD = 3V
RT = 499k, VSYNC/SPRD = 3V

RT Pin Current Limit

VRT = 0V, Current Out of Pin

l
l

1880
175

2000
200

1880
175

kHz
kHz

2650
306

kHz
kHz

75

SYNC/SPRD Threshold (Rising)

1.4

SYNC/SPRD Falling Hysteresis

0.2

SYNC/SPRD Pin Current

2120
245

VSYNC/SPRD = 5V

−100

µA
1.5

V
V

100

nA

Soft-Start
SS Pin Charging Current

VSS = 1V

20

µA

SS Pin Discharging Current

VSS = 2V2µA

SS Lower Threshold

0.2

V

SS Higher Threshold

1.7

V

Fault Detection
Open-Circuit Threshold (FB Rising)

VISP = VISN = 20V

l

1.117

Open-Circuit Falling Hysteresis

1.140

1.163

50

LED Short-Circuit Threshold (VISP − VISN)

VISP = 20V

FAULT Pull-Down Current

VFAULT = 0.2V, VFB = 1.25V

0.8

FAULT Leakage Current

VFAULT = 3V, VFB = 0.7V

−100

V
mV

150

mV
mA
100

nA

1.292

V

Overvoltage Protection
FB Overvoltage Threshold (Rising)

l

1.240

FB Overvoltage Falling Hysteresis

1.266
22

mV

LED Current Monitor
ISMON Voltage

VISP − VISN = 100mV (100%), VISP = 24V
VISP − VISN = 10mV (10%), VISP = 24V

0.980
80

1.000
100

1.020
120

10

11

V
mV

PWM Driver
PWMTG Gate Drive (VOUT – VPWMTG)

VOUT = 20V, VPWM = 1.5V

V

PWM Threshold (Rising)

1.4

V

PWM Falling Hysteresis

0.2

V

PWM Pin Current

VPWM = 2V

PWM to PWMTG Propagation Delay
Turn-On
Turn-OffCPWMTG = 2.1nF (Connected from VOUT to PWMTG)
VOUT = 20V

−100

100
110
140

nA
ns
ns

Rev. A

4

For more information www.analog.com

LT3922-1
ELECTRICAL CHARACTERISTICSPARAMETERCONDITIONSMINPWM Voltage for 100% PWM Dimming

RP = 28.7k, VREF = 2V

2.00

PWM Voltage for 0% PWM Dimming

RP = 28.7k, VREF = 2V

PWM Dimming Accuracy

RP = 28.7k, VREF = 2V, VPWM = 1.1V
RP = 28.7k, VREF = 2V, VPWM = 1.9V

7.5
87.5

PWM Dimming Frequency

RP = 28.7k, RT = 45.3k, VSYNC/SPRD = 0V
RP = 332k, RT = 45.3k, VSYNC/SPRD = 0V

7.34
115

RP Pin Current Limit

VRP = 0V, Current Out of Pin

TYP

MAX

UNITS

Internal PWM Dimming

Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: Do not apply a positive or negative voltage source to these pins,otherwise permanent damage may occur.
Note 3: The LT3922E-1 is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the −40°C
to 125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The

V
0.99

V

10.5
90.5

13.5
93.5

%
%

7.81
122

8.28
129

kHz
Hz

65

µA

LT3922I-1 is guaranteed to meet performance specifications over the−40°C to 125°C operating junction temperature range. The LT3922H-1 isguaranteed over the −40°C to 150°C operating junction temperature range.
Operating lifetime is derated at junction temperatures greater than 125°C.
Note 4: This IC includes overtemperature protection that is intended toprotect the device during momentary overload conditions. The maximum
rated junction temperature will be exceeded when this protection is active.
Continuous operation above the specified absolute maximum operating
junction temperature may impair device reliability or permanently damage
the device.

Rev. AFor more information www.analog.com

5LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS VIN = 12V, TA = 25°C, unless otherwise noted.
EN/UVLO Thresholds
RISINGFALLING
EN/UVLO PIN CURRENT (µA)

1.45
EN/UVLO THRESHOLD (V)EN/UVLO Pin Current
3.00

1.40
1.35
1.30
1.25
1.20
–45 –20

5

VEN/UVLO = 1.2V

2.00
1.50
1.00

5

0.50

3.2

3.20
3.00
2.80
2.60
2.40

5

2.00
–45 –2030 55 80 105 130 155
TEMPERATURE (°C)

5

2.02

–40°C
25°C
125°C
150°C

2.85
2.80

VREF Voltage

0

2

6
8
10
INTVCC LOAD CURRENT (mA)

1239221 G07

025 50 75 100 125 150
TEMPERATURE (°C)

2.00

1.99

1.97
–50 –25

VREF Load Regulation

2.01

1.98

4

2.7

2.02

VREF VOLTAGE (V)

2.90

2.8

39221 G06

2.01
VREF VOLTAGE (V)

INTVCC VOLTAGE (V)

3.05

2.95

2.9

39221 G05

INTVCC Load Regulation

3.00

3.0

2.5
–50 –25

30 55 80 105 130 155
TEMPERATURE (°C)

39221 G04

3.10

VIN = 36V
VIN = 12V
VIN = 2.7V

2.6

2.20
0
–45 –2030 55 80 105 130 155
TEMPERATURE (°C)

INTVCC Voltage

3.1
INTVCC VOLTAGE (V)

1.00

5

39221 G03

VIN = 36V
VIN = 12V
VIN = 2.7V

3.40

1.50

1.15

1.05
–45 –2030 55 80 105 130 155
TEMPERATURE (°C)

VIN Quiescent Current
3.60
VIN QUIESCENT CURRENT (mA)

VIN SHUTDOWN CURRENT (µA)

2.00

1.20

39221 G02

VIN Shutdown Current
VIN = 36V
VIN = 12V
VIN = 2.7V

1.25

1.10

0.50

39221 G01

2.50

RISINGFALLING

1.30

2.50

0
–45 –2030 55 80 105 130 155
TEMPERATURE (°C)

OVLO Threshold
1.35

OVLO THRESHOLD (V)

1.50

2.00

1.99
–40°C
25°C
125°C
150°C

1.98

0

25 50 75 100 125 150
TEMPERATURE (°C)
39221 G08

1.97

0

200
400
600
800
VREF LOAD CURRENT (µA)

100039221 G09

Rev. A

6

For more information www.analog.com

LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS VIN = 12V, TA = 25°C, unless otherwise noted.

2.008
THRESHOLD VOLTAGE (V)

2.006

2.002
2.000
1.998
1.996
1.994

2.7

25

2.4
2.1
1.8
1.5
1.2

1.992
1.990

30

0

3

6

0.9
–45 –20

9 12 15 18 21 24 27 30 33 36
VIN VOLTAGE (V)539221 G10

100
–45 –2030 55 80 105 130 155
TEMPERATURE (°C)

Soft-Start Currents

75

20

1.6

60

THRESHOLD VOLTAGE (V)

2.0

65

15
PULL-UP CURRENT
PULL-DOWN CURRENT
10

5

55
–45 –20

RT PIN
RP PIN5

0–45 –2030 55 80 105 130 155
TEMPERATURE (°C)

5

Switching Frequency

0.8

0.4

250

2000

240

1900

230

1800

220

1700

210

1600

200
190

180
30 55 80 105 130 155
TEMPERATURE (°C)

8320
INTERNAL PWM FREQUENCY (Hz)2100

RT = 45.3k
RT = 499k

530 55 80 105 130 155
TEMPERATURE (°C)
39221 G15Internal PWM Frequency
260

5

SS HIGH
SS LOW

39221 G14

2200

1400
–45 –20

1.2

0–45 –2030 55 80 105 130 155
TEMPERATURE (°C)

39221 G131500

30 55 80 105 130 155
TEMPERATURE (°C)

Soft-Start Thresholds

25

70

5

39221 G1280

CURRENT (µA)

MAXIMUM PIN CURRENT (µA)

1539221 G11

RT and RP Pin Current Limits

SWITCHING FREQUENCY (kHz)20

5INTVCC UVLO
VREF UVLOInternal PWM Duty Cycle

RT = 45.3k

134

7936

7552

128

7168

122

116

6784

6400
–45 –20

RP = 28.7k
RP = 332k
5

10.5

140

110
30 55 80 105 130 155
TEMPERATURE (°C)
39221 G17

39221 G16

INTERNAL PWM DUTY CYCLE (%)

VOLTAGE (V)

2.004

Minimum Off Time

3.0

MINIMUM OFF TIME (ns)2.010

INTVCC and VREF UVLO FallingThresholds

VREF Line Regulation

RT = 45.3k
RP = 332k

10.4

18.2k
1%

10.3

22.1k
1%

VREF
PWM
LT3922-1

10.2

10.1

10.0
–45 –20530 55 80 105 130 155
TEMPERATURE (°C)
39221 G18

Rev. AFor more information www.analog.com

7

LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS VIN = 12V, TA = 25°C, unless otherwise noted.
LED Current (Digital CTRL)100
VISP – VISN (mV)

100
VISP – VISN (mV)

125

75

50

PULSE FREQUENCY AT CTRL PIN = 20kHz

00.25 0.50 0.75 1 1.25 1.50 1.75
VCTRL (V)

75

50

0

2

0

LED Current (5% Regulation)

N = 355

150°C
25°C
–40°C

160

200
150
100

99.3
100
100.7
ISP–ISN VOLTAGE (mV)
VCTRL = 2V
VISP = 24V

120

80

39221 G22

2.9

4.6
5
5.4
ISP–ISN VOLTAGE (mV)

39221 G21400

LED Current Line Regulation
400
PEAK SW CURRENT LIMITED

350

0

20

39221 G23

400

fSW = 2MHz
L = 4.7µH

1.22

600

0

5.8

VCTRL = 0.3V
VISP = 24V

Peak SW Current Limit
3.0

1.21

200

0
4.2

101.4

1.19
1.20
VFB (V)

–40°C
25°C
125°C

800

N = 355

40

50
0
98.6

1.18

ISMON Voltage
1000

ISMON (mV)

250

0
1.17

12.5 25 37.5 50 62.5 75 87.5 100
DCTRL (%)

200

NUMBER OF UNITSNUMBER OF UNITS

150°C
25°C
–40°C

50

39221 G20

LED Current (100% Regulation)
300

75

25

39221 G19350

VCTRL = 2V

100

25

25

0

LED Voltage Limit
125

VISP – VISN (mV)

LED Current (Analog CTRL)
125

40
60
VISP – VISN (mV)

80

10039221 G24

LED Current vs VOUTVFB OVERVOLTAGE PROTECTION LIMITED

350

300

300

2.6

250

250

2.5
2.4
2.3

200
150

2.2

20

30

40 50 60 70
DUTY CYCLE (%)

80

90 10039221 G25

50200
VIN = 8V
fSW = 2MHzRSNS = 0.3Ω
RFB_TOP = 1M
RFB_BOT = 34.8k

150fSW = 2MHzRSNS = 0.3Ω
11 LEDs (VOUT ~33V)

100

10 LEDs (VOUT ~ 30V)
5 LEDs (VOUT ~ 15V)

2.1
2.0
10

ILED (mA)

2.7
ILED (mA)PEAK SW CURRENT (A)

2.8

0

3

6

9

12 15 18 21 24 27 30
VIN (V)39221 G26

100
50

9

15

21
27
VOUT (V)33

3939221 G27

Rev. A8

For more information www.analog.com

LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS VIN = 12V, TA = 25°C, unless otherwise noted.
VISP – VISN SHORTLED Threshold

FB OPENLED Threshold

1.10
1.05
1.00
0.95

5

170
160
150
140
130
120
–45 –2030 55 80 105 130 155
TEMPERATURE (°C)

539221 G28

96

94

94

90
88
86

80

fSW = 400kHzfSW = 2MHz
6

92
90
88
86

80

PWMTG ON Voltage

0

200

800

1.20

1.18
–45 –20

1000

9

8

120

80
CPWMTG = 2.2nF (C0G type)

30 55 80 105 130 155
TEMPERATURE (°C)
39221 G34

530 55 80 105 130 155
TEMPERATURE (°C)
39221 G33FB OVLO Threshold
1.33

160

40
–45 –20539221 G32

TURN OFFTURN ON
PROPAGATION DELAY (ns)VOUT – VPWMTG (V)400
600
ILED (mA)

200

1030 55 80 105 130 155
TEMPERATURE (°C)

1.21

PWM Driver Propagation Delay

12

11

5

1.19

6 LED (VOUT ~ 18V)
9 LED (VOUT ~ 27V)
12 LED (VOUT ~ 36V)

39221 G31

5

50

1.22

82

8 10 12 14 16 18 20 22 24 26 28 30
VIN (V)7–45 –20100

Regulated FB Voltage

VIN = 12V
fSW = 2MHz

84

84
82

150

1.23

FB VOLTAGE (V)

96

92

200

39221 G30

Efficiency vs ILED98

EFFICIENCY (%)

EFFICIENCY (%)

100

ILED = 400mA
11 LEDs (VOUT ~ 33V)

98

250

39221 G29

Efficiency vs VIN
100

TOP SWITCH
BOTTOM SWITCH

300

0–45 –2030 55 80 105 130 155
TEMPERATURE (°C)

30 55 80 105 130 155
TEMPERATURE (°C)
39221 G35FB OVLO THRESHOLD VOLTAGE (V)0.90–45 –20

POWER SWITCH ON–RESISTANCE (mΩ)

RISINGFALLING

1.15

Power Switch On-Resistance350

180
VISP – VISN SHORTLED THRESHOLD (mV)FB OPENLED THRESHOLD (V)1.20

RISINGFALLING

1.30

1.27

1.24

1.21

1.18
–45 –20530 55 80 105 130 155
TEMPERATURE (°C)
39221 G36

Rev. AFor more information www.analog.com

9LT3922-1
TYPICAL PERFORMANCE CHARACTERISTICS VIN = 12V, TA = 25°C, unless otherwise noted.
C/10 Threshold

Case Temperature Rise
35

16
14
12
10
8
6
–45 –20530 55 80 105 130 155
TEMPERATURE (°C)

25

15
10
5
0

150°C
25°C
–40°C

8

12

16
20
VIN (V)

24

28

N = 359

250
200
150
100
50

fSW = 2MHzfSW = 400kHz
4

150°C
25°C
–40°C

300

20

Internal PWM Duty Cycle (90%)
250

DC2247A DEMO BOARD
VLED = 30V
I LED = 333mA
TROOM = 25°C

3039221 G37300

Internal PWM Duty Cycle (10%)
350

NUMBER OF UNITSRISINGFALLINGCASE TEMPERATURE RISE (°C)

VISP – VISN C/10 THRESHOLD (mV)

18

32

0
8.6

9.4
10.2
11.0
PWM DUTY CYCLE (%)

11.8
39221 G39

39221 G38

Input Voltage Transient Response

Input Voltage Transient Response

N = 359
VIN
5V/DIV

VIN
5V/DIVILED
100mA/DIV

ILED
100mA/DIV

200
150
100
5ms/DIV50
090.2 90.6

91.4
92.2
PWM DUTY CYCLE (%)

93.0 93.4

FRONT PAGE APPLICATION
18V to 6.5V INPUT VOLTAGE TRANSIENT
VLED = 30V
ILED = 333mA

Start-Up with 10% Internal PWM

Start-Up with 50% Internal PWM

VOUT
10V/DIV

VIN
10V/DIV

VOUT
10V/DIV

VIN
10V/DIVILED
100mA/DIV

VIN
10V/DIVILED
100mA/DIV

ILED
100mA/DIV

500µs/DIVFRONT PAGE APPLICATIONVLED = 30V
ILED = 333mA

39221 G43

39221 G42

FRONT PAGE APPLICATION
6.5V to 18V INPUT VOLTAGE TRANSIENT
VLED = 30V
ILED = 333mA

39221 G40

Turn ON and OFF Performance

5ms/DIV

39221 G41

39221 G44

5ms/DIVFRONT PAGE APPLICATION WITH PWM = 1.1V
RP = 332k TO GND
VLED = 30V
ILED = 333mA

39221 G45

5ms/DIVFRONT PAGE APPLICATION WITH PWM = 1.5V
RP = 332k TO GND
VLED = 30V
ILED = 333mA

Rev. A

10

For more information www.analog.com

LT3922-1
PIN FUNCTIONS
SW: Switch Pins. These pins are internally connected tothe power devices and drivers. They should always betied together. In normal operation, the voltage of thesepins will switch between the output voltage and zero at
the programmed frequency. Do not force any voltage onthese pins.
BST: Boost Pin. This pin supplies the top power switch
GATE driver. Connect a 33nF capacitor between this pin
and SW close to the package. An internal diode fromINTVCC to BST will charge the capacitor when the SW pin
switches low.
INTVCC: Internally Regulated, Low-Voltage Supply Pin.
This pin provides the power for the converter switch GATE
drivers. Do not force any voltage on this pin. Place a 2.2µF
bypass capacitor to GND close to the package.
VIN: Input Voltage Pin. This pin supplies power to theinternal, high-performance analog circuitry. Connect a
bypass capacitor between this pin and GND.
EN/UVLO: Enable and Undervoltage Lockout Pin. A voltage at this pin greater than 1.33V will enable switching,
and a voltage less than 0.1V is guaranteed to shut down
the internal current bias and sub-regulators. A resistor
network between this pin and ground can be used to set
the pin voltage and automatically lockout the part when
VIN is below a certain level. No internal components pull up
or down on this pin, so it requires an external voltage bias
for normal operation. This pin may be tied directly to VIN.
OVLO: Input Overvoltage Lockout Pin. When the voltage at
this pin rises above 1.205V, the system disables switching and resets the soft-start capacitor. Do not leave thispin open. Tie this pin to GND when the OVLO function is
not used.
VREF: Reference Voltage Pin. This pin provides a buffered
2V reference capable of 3mA drive. It can be used to supply
resistor networks for setting the voltages at the CTRL andPWM pins. Bypass with a 1μF capacitor to GND.
CTRL: Control Pin. An analog voltage from 250mV to
1.25V at this pin programs the regulated voltage between

ISP and ISN (and therefore, the regulated current supplied
to the load). Alternatively, a digital pulse at this pin withduty cycle from 12.5% to 62.5% can be used to program
the regulated voltage. Below 200mV or 10% duty cycle,
the CTRL pin voltage disables switching. For more detail,see Typical Performance Characteristics and Applications
Information sections.
ISP: Positive Current Sense Pin. This pin is one of the inputs
to the internal current sense error amplifier. It should beconnected to the positive side of the external sense resistor. Use Kelvin connection for accurate current sensing.
ISN: Negative Current Sense Pin. This pin is one of the
inputs to the internal current sense error amplifier. It
should be connected to the negative side of the externalsense resistor. Use Kelvin connection for accurate current sensing.
VC: Compensation Pin. A resistor and capacitor connected
in series from this pin to GND stabilize the current andvoltage regulation. Typical resistor and capacitor values
are from 0k to 100k and from 0.1nF to 10nF, respectively.
FB: Feedback Pin. When the voltage at this pin is near 1.2V
the regulated current is automatically reduced from theprogrammed value. A resistor network between this pin
and VOUT can be used to set a limit for the output voltage.
If the voltage at the FB pin reaches 1.266V, a FB overvoltage lockout comparator disables switching.
SS: Soft-Start Pin. At startup and recovery from fault
conditions, a 20μA current charges the capacitor and theFB voltage tracks the rising voltage at this pin until the load
current reaches its programmed level. Typical values forthe capacitor are 10nF to 100nF. Using a single resistor
from SS to INTVCC, the LT3922-1 can be set in two different fault modes for the shorted LED conditions: hiccup(no resistor) and latchoff (100k). Refer to the Applications
Information section for a detailed explanation.
ISMON: Output Current Monitoring Pin. This pin provides
a buffered voltage output equal to 10mV for every 1mV
between ISP and ISN.

Rev. AFor more information www.analog.com

11

LT3922-1
PIN FUNCTIONSFAULT: Fault Pin. Connect to INTVCC through a resistanceof 100k. An internal switch pulls this pin low when any offollowing conditions happen:
1. Open LED: VFB > 1.14V and (VISP – VISN) < 10mV
2. Shorted LED:
(VISP – VISN) > 150mV for more than 300us, or(VISP – VISN) > 700mV (typical), or
VOUT < (VIN – 2V)
RT: Timing Resistor Pin. A resistor from this pin to GND
programs the switching frequency between 200kHz and2MHz. Do not leave this pin open.
SYNC/SPRD: Synchronization Pin. To override the programmed switching frequency, drive this pin with an
external clock having a frequency between 200kHz and2MHz. Even when using the external clock, select an RT
resistor that corresponds to the desired switching frequency. Tie the pin to INTVCC to enable Spread Spectrum
Frequency Modulation. This pin should be tied to GND
when not in use.
RP: PWM Resistor Pin. Connect a resistor from this pin to
GND to set the frequency of the internal PWM signal. Do not
use a resistor larger than 1MΩ. If using an external PWM

pulse for LED dimming, tie this pin to GND. Refer to theApplications Information section for a detailed explanation.
PWM: PWM Input Pin. With the RP pin tied to GND, drive
this pin with a digital pulse to control PWM dimming of
the LEDs. Alternatively, when using a resistor on the RP
pin to GND, set the voltage of this pin between 1V and
2V to generate an internal pulse with duty cycle between
0% and 100%. When using an analog signal, place a 1µF
bypass capacitor between this pin and GND. Tie this pin
high when PWM dimming is not required.
PWMTG: PWM Driver Output Pin. This pin can drive thegate of an external high-side PMOS device for PWM dimming of LEDs. Do not force any voltage on this pin.
VOUT: Output Pins. Connect to the output and place output
capacitors between these pins and GND as close as possible to the package. Refer to the Applications Information
section for the recommended capacitor placements.
GND (Pin 22, 23, Exposed Pad Pin 29): Ground Pins. All
GND pins must be soldered to the board ground plane.
NC: No Connect Pins. These pins can be left open or connected to the ground.

Rev. A

12

For more information www.analog.com

LT3922-1
BLOCK DIAGRAM
VIN

4.7µH
10µF

2.2µF
3

VIN

2

INTVCC

33nF
BST

SW

28

27

EN/UVLO

INTERNAL VCC
REGULATOROVLOUVLO AND
OVLO

5
356k

MTSW
S
R

6
PEAK CURRENTCOMPARATOR

VREF
2V REFERENCE

VOUT

Q

24

SYNCHRONOUS
CONTROLLER

59.0k

7

29
GND
(EXPOSED PAD)

4
1M

1

GND
MBSW

RT
16
45.3k
1.4V

–

SYNC/SPRD
17+

0.47µF

VOUT

200kHz TO
2MHz
OSCILLATOR

4.7µF

23

+
–
+

1µF

0.47µF
22

200mΩ

21
VOUT – 10V
REGULATOR
PWMTG

V/I
CONVERTER

PWMTG
DRIVER20

gm = 140µA/V
CURRENTREGULATIONAMPLIFIER

+

–

PWM
19

2.5k

2.5k

ISMON

INTERNAL
PWM SIGNAL

14

25k
0.25V

10×
gm = 1000µA/V
VOLTAGEREGULATIONAMPLIFIER–

A/D
DETECTORCONTROL
BUFFER

+

8

+
+
–

ISN
10
ISP
9

+
+
–

CTRL

+

–

1.25V

12

1.2V

INTVCC

20µA
FAULT
SOFT-START
AND LEDFAULT CONTROL

2µA
SS
25
N/C

26
N/C

1M

FB

13

VC
11

10nF

33.2k

100k

15

RP
18

39221 BD

10k
1nF

Rev. AFor more information www.analog.com

13

LT3922-1
OPERATION
The LT3922-1 is a step-up LED driver that utilizes a fixedfrequency peak current control to accurately regulate thecurrent through a string of LEDs. Low EMI performance isrealized with the LT3922-1's Silent Switcher architecture,
which employs magnetic field cancellation techniques tominimize electromagnetic interference. The LT3922-1
includes two power switches, their drivers, and a diodefor providing power to the top switch driver. The switches
connect the external inductor terminal connected to theSW pin alternately to the ground and then to the output(VOUT). The inductor current rises and falls accordingly andthe peak current can be regulated through the combined
effect of the other circuit blocks.
The synchronous controller ensures the power switches
do not conduct at the same time, and a programmableoscillator turns on the bottom switch at the beginning ofeach switching cycle. The frequency of this oscillator is set
by an external resistor at the RT pin and can be overridden by external pulses at the SYNC/SPRD pin. The SYNC/SPRD pin can also be used to command spread spectrum
frequency modulation (SSFM).
The bottom switch is turned off by the peak current comparator which waits during the on-time for the inductor
current to exceed the target set by the voltage at the VC
pin. This target is modified by a signal from the oscillator
which stabilizes the inductor current. A network of passive components at the VC pin is necessary to stabilize
this regulation loop.
The inductor current is derived from the desired LED
current programmed by the voltage at the CTRL pin. The
analog-to-digital detector and the control buffer convert
either a DC voltage or duty cycle of pulses at the CTRL

pin into the input for the current regulation amplifier. The
other input to this amplifier comes from the ISP and ISN
pin voltages. An external current sense resistor between
these pins should be placed in series with the string ofLEDs such that the voltage across it provides the feedback
to regulate the LED current. The current regulation amplifier then compares the actual LED current to the desired
LED current and adjusts VC as necessary.
The voltage regulation amplifier overrides the current
regulation amplifier when the FB pin voltage is higher thanan internal 1.2V reference. An external resistor networkfrom the LED string to the FB pin provides an indicationof the LED string voltage and allows the voltage amplifier
to prevent overvoltage of the LED string.
The ISP, ISN, and FB pin voltages are also monitored todetect fault conditions like open and short circuits, which
are then reported by pulling FAULT pin low. The response
to a fault can be selected either to try hiccup restarts or tolatchoff by the choice of an external resistor connected to
the SS pin. Refer to the Applications Information section
for a detailed explanation of fault responses.
Finally, pulse-width modulation (PWM) of the LED current
is achieved by turning on and off an external PMOS switch
between the VOUT and the string of LEDs. An external pulse
at the PWM pin controls the state of the PWM driver or
a DC voltage at the PWM pin dictates the duty ratio of aninternal PWM pulse, whose frequency is programmedwith an external resistor at the RP pin. The proprietarycircuits of the LT3922-1 ensures a rapid recovery of the
LED current pulses for PWM.

APPLICATIONS INFORMATION
The following is a guide to selecting the external components and configuring the LT3922-1 according to therequirements of an application.
Programming LED Current with the CTRL Pin
The primary function of the LT3922-1 is to regulate thecurrent in a string of LEDs. This current should passthrough a series current sense resistor. The voltage acrossthis resistor is sensed by the current regulation amplifier
through the ISP and ISN pins and regulated to a level programmed by the CTRL pin. The maximum resistor voltagethat can be programmed is 100mV which corresponds to
1A through the LED string when a 100mΩ current sense
resistor is used.
To allow for this maximum current, the CTRL pin may
be connected directly to the VREF pin which provides

Rev. A

14

For more information www.analog.com

LT3922-1
APPLICATIONS INFORMATIONan accurate 2V reference. Lower current levels can be
programmed by DC CTRL voltages between 250mV and
1.25V as shown in Figure 1.

ILED
100mV
RSNS
DCTRL < 10%
CTRL-OFF

ILED
100mV
RSNS
50mV
RSNS

VCTRL < 200mV
CTRL-OFF

50mV
RSNS
0

12.5%

37.5%

62.5%

75%

DCTRL
39221 F02

Figure 2. Duty Cycle CTRL Range
0

0.25V

0.75V

1.25V

1.5V

VCTRL

VREF

VREF

39221 F01

RCTRL1Figure 1. Analog CTRL Range

LT3922-1

RCTRL1

Below 250mV, the CTRL pin commands zero LED current,
and above 1.25V, it commands the maximum. When an
independent voltage source is not available, the intermediate CTRL voltages may be derived from the 2V referenceat the VREF pin using a resistor network or potentiometeras long as the total current drawn from the VREF pin isless than 1mA.
Additionally, the LT3922-1 is capable of interpreting a
digital pulse at the CTRL pin. The high level of the pulse
must be greater than 1.6V. The low level must be lessthan 0.4V. The frequency must be greater than 10kHz andless than 200kHz. Then the regulated voltage between
ISP and ISN will vary with the duty ratio of the pulse as
shown in Figure 2.
In this case, the LED current is zero for duty cycles lessthan 12.5% and reaches its maximum above 62.5%. The
LT3922-1 will cease switching if the duty cycle of the
CTRL pin pulse is less than 10%, and also for DC CTRL
pin voltages less than 200mV.

RNTC

LT3922-1
CTRL

CTRL
RCTRL2RCTRL239221 F03

RNTC

Figure 3. Setting CTRL with NTC Resistors

Setting Switching Frequency with the RT Pin
The switching frequency of the LT3922-1 is programmed
by a resistor connected between the RT pin and GND.
Values of the RT resistor from 45.3k up to 499k program
frequencies from 2MHz down to 200kHz as shown in
Table 1. Higher frequencies allow for smaller external
components but increase switching power losses andradiated EMI.
Table 1. RT Resistance Range
SWITCHING FREQUENCY

RT

2.0 MHz

45.3k

1.6 MHz

57.6k

1.2 MHz

78.7k

1.0 MHz

95.3k

400 kHz249k

200 kHz499k

To reduce the LED current when the temperature of theLEDs rises, use resistors with negative temperature coefficient (NTC) in the network from VREF to CTRL as shownin Figure 3.

Rev. AFor more information www.analog.com

15

LT3922-1
APPLICATIONS INFORMATION
Synchronizing Switching Frequency

Maximum Duty Cycle

The switching frequency can also be synchronized toan external clock connected to the SYNC/SPRD pin. The
high-level of the external clock must be at least 1.5V, andthe frequency must be between 200kHz and 2MHz. The
RT resistor is still required in this case, and the resistanceshould correspond to the frequency of the external clock.
If the external clock ever stops, the LT3922-1 will rely on
the RT resistor to set the frequency.

The choice of switching frequency should be made knowing that the maximum VOUT voltage of a boost converter
is determined by the maximum duty cycle for a given VIN
voltage as shown in the following equation:

Enabling Spread Spectrum Frequency Modulation
Connecting SYNC/SPRD to INTVCC will enable spread
spectrum frequency modulation (SSFM). The switchingfrequency will vary from the frequency set by the RT resistor
to 125% of that frequency. If neither synchronization nor
SSFM is required, connect SYNC/SPRD to GND.
As shown in Figure 4, enabling SSFM can significantly attenuate the electromagnetic interference that the LT3922-1,
like all switching regulators, emits at the switching frequency and its harmonics. This feature is designed to help
devices that include the LT3922-1 perform better in thevarious standard industrial tests related to interference.
80
SSFM ON
SSFM OFFPEAK CONDUCTED EMI (dBµV)

70
60
50
40
30
20
10
0
–10
–20

0

5

10

15
20
FREQUENCY (MHz)253039221 F04

Figure 4. Typical Conducted Peak EMI of the LT3922-1 with
2MHz Switching Frequency

The attenuation varies depending on the chosen switchingfrequency, the range of frequencies in which interference ismeasured, and whether a test measures peak, quasi-peak,or average emissions. The results of several other emission
measurements are with select typical application circuits.

VOUT =

VIN(1)( 1– D )where D is the duty cycle of the boost converter definedas the ratio of the on-time of the bottom power switch to
the total switching period. The maximum duty cycle for agiven switching frequency is determined by the minimum
off-time of the bottom power switch. The longest minimum
off-time of the LT3922-1 is 35ns, so the maximum duty
cycle is 93% at 2MHz switching frequency. Therefore, if
an application requires higher duty cycle, the switchingfrequency should be set lower to achieve the demanded
duty cycle.
Selecting an Inductor
The LT3922-1 limits the inductor peak current to a minimum of 2.3A over the duty cycle without sub-harmonic
oscillations. This current limit will override the CTRL input
command if the programmed LED current demands higher
inductor peak current than 2.3A. Therefore, it is important toselect the inductor value to ensure the peak inductor current
is below the limit over the desired input voltage range. The
following is an example of inductor value decision process
for the application where we want 300mA LED current at
30V output, while the input ranges from 8V to 25V and theswitching frequency is 2MHz. The maximum peak inductor
current can be derived by adding the half of the inductor
current ripple amplitude to the average inductor current
value, both values of which are determined by the input
and output voltages, switching frequency, efficiency andthe inductor values. Hence, the minimum inductor value
LMIN that ensures the peak inductor current below 2.3A is:()⎛ VIN(MIN) • VOUT – VIN(MIN) ⎞⎜⎟2 • VOUT • fSW⎜⎝⎟⎠
LMIN =
VOUT • ILED⎛⎞⎜⎝ 2.3 – V⎟
IN(MIN) • EFFICIENCY ⎠(2)
Rev. A

16

For more information www.analog.com

LT3922-1
APPLICATIONS INFORMATION
Using this equation gives an inductance of about 1.4µH
assuming 90% efficiency for the given conditions.
With this minimum inductor value guideline, choose an
inductor with low core loss and low DC resistance. Inductor must be able to handle the peak inductor current
without saturation. To minimize the radiated noise, use ashielded inductor. The manufacturers featured in Table 2
are recommended sources of inductors.
Table 2. Inductor Manufacturers
MANUFACTURERWEBSITE

Wurth Electronics

www.we-online.com

Coilcraftwww.coilcraft.com

Vishay Intertechnology

www.vishay.com

Selecting an Input Capacitor
The input capacitor supplies the inductor ripple current
and the transient current that occurs in PWM dimming
operations. A 10µF ceramic capacitor should be sufficient
to provide these non-steady state currents. Place the input capacitor close to the inductor. If possible, place anadditional 1µF ceramic capacitor close to the VIN pin for
better noise immunity. Use X7R or X5R ceramic capacitors
as they typically retain their capacitance better than othercapacitor types over wide voltage and temperature ranges.
If the input power source has high impedance, or there
is significant inductance due to long wires or cables, additional bulk electrolytic capacitance may be necessary. A
low ESR ceramic input capacitor combined with parasitic
inductances in the current paths can form a high-Q LC tankcircuit which can ring the capacitor voltage up to twice
the input voltage. A higher ESR electrolytic capacitor, on
the other hand, minimizes this ringing. Refer to the Linear
Technology Application Note 88 for more information.
Sources of quality ceramic and electrolytic capacitors arelisted in Table 3.

Table 3. Capacitor Manufacturers
MANUFACTURERWEBSITE

Murata Manufacturingwww.murata.com

Garrett Electronics

www.garrettelec.com

Panasonic

www.industrial.panasonic.com

Nippon Chemi-Con

www.chemi-con.co.jp

Stabilizing the Regulation Loop
The LT3922-1 uses internal error amplifiers to regulate
the LED current and the output voltage to the user programmed values. The output impedance of the error
amplifiers and the external compensation capacitor, CC,connected to VC pin create the dominant pole of the control
loop. The compensation resistor, RC, in series with CC
forms a left-half-plane (LHP) zero. This LHP zero allows
better regulation of LED current and output voltage during transient operations. For most LED applications, 1nF
and 10k would be good starting values for CC and RC,
respectively. Refer to the Linear Technology Application
Note 76 for more information.
Selecting and Placing Output Capacitors
The output capacitors need to have very low ESR to reduce the output ripple. Placing several low ESR ceramic
capacitors in parallel is an effective way to reduce ESR.
These output capacitors in a boost converter should have aripple current rating greater than the half of the maximum
SW pin current. Use X7R or X5R ceramic capacitors as
they typically retain their capacitance better than othercapacitor types over wide voltage and temperature ranges.
The LT3922-1 utilizes a proprietary architecture to reduce
EMI noise generated by switching. To best utilize thisfeature, VOUT should be bypassed with three capacitors.
Figure 5 shows the VOUT capacitor placements for the
QFN package. COUT1 and COUT2 are 0402-0.47µF ceramic
capacitors placed as close as possible to the LT3922-1’s
VOUT and GND pins. COUT3 should be larger in size andvalue. A 1206-(2.2µF to 22µF) ceramic capacitor is recommended for typical applications.

Rev. AFor more information www.analog.com

17

LT3922-1
APPLICATIONS INFORMATION
The drain source voltage rating of the chosen PMOS
should be greater than the maximum output voltage. Typically the output voltage is a little higher than the sum of
the forward voltages of the LEDs in the string. However,when the string is broken, the output voltage will begin to
increase due to the imbalance of inductor current and load
current. As described in detail later, the LT3922-1 will not
reduce the inductor current nor limit the output voltage
until the FB pin voltage approaches 1.2V. Therefore, themaximum output voltage is ultimately determined by theresistor network between FB and VOUT.

VOUT
VOUT

COUT3
1206

COUT1
0402

25

24

VOUT

23
22

COUT2
0402

21
GND

20
39221 F05

Figure 5. Placement of Output Capacitors

Selecting a MOSFET for PWM Dimming
Pulse-Width-Modulation (PWM) dimming of the LED current is an effective way to control the brightness of thelight without varying its color. The brightness can also
be adjusted with finer resolution this way than by varying
the current level.
The LT3922-1 features a PWMTG driver that is intended
for a high-voltage PMOS switch in position to effectively
PWM dim a string of LEDs from the output capacitor andthe current sense resistor. When the switch is open andthe string is disconnected, the LED current will be zero. In
contrast to a low-side NMOS driver, this feature eliminates
the need for a dedicated return path for the LED current
in automotive applications or other grounded chassis
systems.
The gate driver for this PMOS is supplied through the VOUT
pin. When the PWM pin voltage is greater than 1.4V, thedriver will pull the gate of the PMOS to a maximum of 10V
below the VOUT pin. If VOUT is below 10V, the gate driveis necessarily reduced. For constant current applications,leave PWMTG open, connect the load directly after thecurrent sense resistor, and connect PWM to INTVCC. In
these cases, analog dimming may be implemented with
the CTRL pin.

In most applications, the gate source voltage rating of the
PMOS should be at least 10V. The only exceptions to thisrule are applications for which the output voltage is always
less than 10V. The PWMTG driver will try to pull the gateof the PMOS down to 10V below VOUT, but it cannot pull
the gate below GND. Therefore, when the maximum output
voltage is less than 10V, the PMOS gate source voltage
rating will be sufficient if it is merely equal to or greaterthan the output voltage.
Finally, the drain current rating of the PMOS must exceed
the programmed LED current. Assuming this condition andthe conditions above are met, the only electrical parameterto be considered is the on-resistance. Other parameterssuch as gate charge are less important because PWM dimming frequencies are typically too low for efficiency to beaffected noticeably by gate charging loss or transition loss.
Table 4 lists recommended manufacturers of PMOS
devices.
Table 4. PMOS Manufacturers
MANUFACTURERWEBSITE

Infineon

www.infineon.com

Vishay Intertechnology

www.vishay.com

Fairchild Semiconductor Corp.

www.fairchildsemi.com

NXP Semiconductors

www.nxp.com

Selecting an RP Resistor for Internal PWM Dimming
If the RP pin is tied to GND, an external pulse-widthmodulated signal at the PWM pin will control PWM dimming of the LED load. The signal will enable the PWMTG
driver and turn on the external PMOS device when it ishigher than 1.4V.
Rev. A18

For more information www.analog.com

LT3922-1
APPLICATIONS INFORMATION
However, the LT3922-1 is capable of PWM dimming evenwhen an external PWM signal is not available. In this case,an internal PWM signal with frequency set by a resistor at
the RP pin and duty ratio set by a DC voltage at the PWM
pin will control the PWMTG driver. The RP resistor should
be one of the seven values listed in Table 5. For each ofthese values, the PWM frequency is a unique ratio of theswitching frequency.

High PWM Dimming Ratio

Table 5. Internal PWM Dimming Frequencies

The ISMON pin provides an amplified and buffered monitor
of the voltage between the ISP and ISN pins. The gain of
the internal amplifier is ten, and the speed is fast enoughto track the pulse-width modulated LED current. However,as shown in Figure 6, the ISMON voltage can be filteredwith a resistor-capacitor network to monitor the average
LED current instead.

SWITCHING FREQUENCY
RP

RATIO

2MHz

1MHz

200kHz28.7k

287.81kHz

3.91kHz

781Hz

47.5k

29

3.91kHz1.95kHz

391Hz

76.8k

210

1.95kHz

977Hz

195Hz

118k

211

977Hz

488Hz

97.7Hz

169k

212

488Hz

244Hz

48.8Hz

237k

213

244Hz

122Hz

24.4Hz

332k

214

122Hz

61Hz

12.2HzWhen using the internal PWM signal, set the voltage at
the PWM pin between 1V and 2V. The PWMTG driver will
stay off if PWM is below 1V, and it will stay on if PWM
is above 2V. Between 1V and 2V there are 128 evenly
spaced thresholds corresponding to 128 discrete PWM
duty ratios from 0% to 100%. This range of 1V to 2V has
been chosen so that the PWM voltage may be set using
a potentiometer or a resistor network and the 2V reference available at the VREF pin. Place a small 1µF ceramic
capacitor near PWM pin to ground.
There is one exception to the above rules for PWM dimming. To avoid excessive start-up times, after the first
PWM pulse, PWMTG will stay on until the SS pin voltage
reaches 1.7V or the LED current has reached approximately
10% of the full-scale current.

The LT3922-1 can drive regulated current pulses with aduration as short as 400ns. This means the PWM dimming ratio can be 25,000:1 when the external PWM signal
frequency is 100Hz. The PWM dimming ratio can be evenhigher as there are no limits on the maximum PWM period.
Monitoring LED Current

LT3922-1

RMON

ISMON

ISMON(FILTERED)
CMON
39221 F06

Figure 6. ISMON Filter Configuration

The resistor should be at least 10k. The capacitance canbe as large or small as needed without affecting the stability of the internal amplifier. For example, when the PWM
frequency is 200Hz, a 10μF capacitor combined with the
10k resistor would limit the ripple on ISMON to 1%.
Selecting the FB Resistors
Two resistors should be selected to form a network between
the output voltage and the FB pin as shown in Figure 7.
VOUT
LT3922-1

RFB2

! R $
VOUT(MAX) = 1.2V • # 1+ FB2 &" RFB1 %FB
RFB139221 F07

Figure 7. FB Resistor ConfigurationRev. AFor more information www.analog.com

19

LT3922-1
APPLICATIONS INFORMATIONThis network forms part of a voltage regulation loop when
FB is near 1.2V. In this case, the LT3922-1 will override
the programmed LED current and adjust the inductor current to lower the output voltage and limit FB to 1.2V. This
resistor configuration therefore determines the maximum
output voltage.
In this way, the LT3922-1 can also be configured as a
voltage regulator instead of an LED driver. It will regulate
the output voltage near the programmed maximum as longas the load current is less than the current programmedby CTRL.
Note that this voltage limit may be reached inadvertently
if it is set too close to the typical output voltage and theoutput capacitor is too small. To avoid interference withthe current regulation, the feedback resistors should bechosen such that FB is below 1.14V when the LEDs are
conducting.

Open LED Fault Detection and Response
The resistor network formed by RFB1 and RFB2 also defines
the criteria for the open-LED fault condition. An open-LED
fault is detected when the FB pin voltage is greater than
1.14V and simultaneously the difference between ISP and
ISN pins is less than 10mV. The latter condition ensuresthat the output current is low (as it should be in an open
circuit) not just that output voltage is high as it may bewhen the LEDs are conducting a large current.
A fault is reported by an internal device pulling the voltage
at the FAULT pin low. There is nothing internal that pulls
this voltage high, so an external resistor between INTVCC
and FAULT is necessary as shown in Figure 8. This configuration allows multiple FAULT pins and similar pins onother parts to be connected and share a single resistor.
INTVCCRFAULT

Understanding FB Overvoltage Lockout

LT3922-1
FAULT

Despite the voltage regulation loop, the FB voltage cantemporarily exceed the 1.2V limit. If the output voltage
is near the maximum when the LED string opens, it may
take too long for the feedback loop to adjust the inductor
current and avoid overcharging the output. To quickly
respond to the overvoltage conditions, the LT3922-1 will
immediately stop switching, disconnect the LED string by
shutting the external PMOS off when the FB pin exceeds
the 1.266V FB overvoltage lockout threshold.
The FB overvoltage lockout threshold may be routinely
exceeded when the LT3922-1 is being operated as a voltage regulator if the load current decreases rapidly. In thiscase, the pause in switching limits the output overshootand ensures that the voltage is back in regulation as
quickly as possible. For safe operation, choose RFB1 andRFB2 values to ensure the output voltage is not greaterthan 40V when the FB voltage is 1.266V.

39221 F08

Figure 8. FAULT Resistor Configuration

Shorted LED Fault Detection and Responses
The LT3922-1 prevents excessive currents that could damage the LED and the driver by three detection schemes
as follows:
1) (VISP – VISN) > 150mV for more than 300µs, or2) (VISP – VISN) > 700mV (typical), or
3) VOUT < (VIN – 2V)
If the LT3922-1 detects any one of these events, it immediately stops switching, turns off the external PMOS
PWM switch, pulls down FAULT pin, and initiates a fault
response routine using the SS pin. Note that FAULT pin isheld low until the part successfully restarts.

Rev. A

20

For more information www.analog.com

LT3922-1
APPLICATIONS INFORMATION
Soft-Start and Fault Modes

SS PIN (V)The LT3922-1’s soft-start (SS) pin has two functions. First,
it allows the user to program the output startup voltage
ramp rate through the SS pin. An internal 20µA current
pulls up the SS pin to INTVCC. As shown in Figure 9, connecting an external capacitor CSS at the SS pin to GND will

VINTVCC (3V)

1.7V

INTVCCRSS(OPTION FOR LATCH-OFF)

LT3922-1

TIME

DETECTED LED SHORTSS
CSS(a) Latchoff Mode
39221 F09

Figure 9. SS Capacitor and Resistor Configuration

generate a linear ramp voltage. This voltage ramp at theSS pin forces the LT3922-1 to regulate the FB pin voltage
to track the SS pin voltage until VOUT is high enough todrive the LED at the commanded current level.
The SS pin is also used as a fault timer. After a shorted
LED fault is detected, an internal 2µA current pulls down
the voltage on the SS pin. The user can configure twodifferent fault response routines by using or not using apull-up resistor, RSS, from the SS pin to INTVCC. Figures
10a and 10b illustrate corresponding waveforms of theSS pin voltage for the two responses: latchoff and hiccup
mode. With a 470k or smaller RSS, the LT3922-1 will latch
off until the user forces a reset by toggling the EN/UVLO
pin. Without the RSS, the LT3922-1 enters a hiccup mode
operation. The 2µA pulls SS pin down to 0.2V, at which
point the 20µA pull-up current turns on again to raise theSS pin voltage. If the fault condition has not been removed
until the SS pin reaches 1.7V, the 2µA pull-down current
source turns on again to start another cycle. This hiccup
mode will continue until the fault is cleared. A typical CSS
value is 10nF.
Programming EN/UVLO and OVLO Thresholds
The LT3922-1 will stop switching, disable the PWMTG
driver, and reset the soft-start when the voltage at the
EN/UVLO pin drops below 1.33V, or the voltage at the
OVLO pin rises above 1.205V. External voltage sources
can be used to set the voltage at EN/UVLO and OVLO pins

SS PIN (V)
INTVCC (3V)

1.7V

0.2V
TIME

DETECTED LED SHORTFAULT CLEARED

39221 F10(b) Hiccup Mode
Figure 10. Fault Responses: (a) Latchoff and (b) Hiccup

to enable or disable the LT3922-1. Alternatively, resistor
networks can be placed from VIN to these pins to set theoperating range of VIN voltage.
For instance, the VIN undervoltage lockout (UVLO) threshold can be accurately set by an external resistor divider.
Figure 11 illustrates how to set the falling EN/UVLO
threshold and the rising hysteresis voltages in LT3922-1.
The internal hysteresis is 25mV, but the user can program
VIN

FALLING THRESHOLD

R1

LT3922-1! R1$
VIN(UVLO) = 1.33V • # 1+ &" R2 %

EN/UVLO
RISING HYSTERESIS

R239221 F11

! R1$
VHYST(UVLO) = 25mV • # 1+ & +R1• 2µA" R2 %

Figure 11. EN/UVLO Threshold and Hysteresis Voltages

Rev. AFor more information www.analog.com

21

LT3922-1
APPLICATIONS INFORMATION
additional hysteresis through the external resistor as the
EN/UVLO pin sinks 2µA current when the EN/UVLO pin
voltage is below the threshold.
On the other hand, the VIN overvoltage lockout (OVLO)
threshold can be accurately set by the external resistor
divider as well. Figure 12 illustrates how to set the rising
OVLO threshold in LT3922-1. The internal hysteresis of
the OVLO pin is 50mV.
VIN

RISING THRESHOLD

LT3922-1

R3! R3 $
VIN(OVLO) = 1.205V • # 1+ &" R4 %OVLOFALLING HYSTERESIS

R439221 F12

! R3 $
VHYST(OVLO) = 50mV • # 1+ &" R4 %

Figure 12. OVLO Threshold and Hysteresis Voltages

Both EN/UVLO and OVLO can be set precisely using asingle resistor string consisting of three series resistors.
Figure 13 shows the resistor string and the threshold andhysteresis voltages for EN/UVLO and OVLO.
VIN
R5 $
!
VIN(UVLO) = 1.33V • # 1+
" R6+R7 &%R5EN/UVLO
R6

LT3922-1
OVLO

R739221 F13R5 $!
VHYST(UVLO) = 25mV • # 1+
+R5 • 2µA
" R6+R7 &%! R5+R6 $VIN(OVLO) = 1.205V • # 1+&"
R7 %! R5+R6 $VHYST(OVLO) = 50mV • # 1+
&
"
R7 %

Figure 13. EN/UVLO–OVLO Threshold and Hysteresis Voltages

Tie EN/UVLO to VIN and tie OVLO to GND if they are notused. Do not leave these pins open.
Planning for Thermal Shutdown
The LT3922-1 automatically stops switching when theinternal temperature is too high. The temperature limit isguaranteed to be higher than the operational temperatureof the part. During thermal shutdown, all switching is terminated, SS is forced low, and the LEDs are disconnected
through the PWMTG driver.

The exposed pad on the bottom of the package must be
soldered to a ground plane. Vias placed directly under the
package are necessary to dissipate heat. Following theseguidelines, the official four-layer demo board DC2247A
reduces the thermal resistance, θJA to 25°C/W. With acompromised board design, θJA could be 40°C/W or higher.
Designing the Printed Circuit Board (PCB)
The output capacitors COUT1 and COUT2 of the LT3922-1
bypass large switched currents from VOUT to GND (seeFigure 5). The loops travelled by these currents should
be made small as possible to these pins. These output
capacitors, along with the inductor and the input capacitors, should be placed on the same side of the PCB, andtheir connections should be made on that layer.
Create a Kelvin ground network by keeping the groundconnection for all of the other components separate.
It should only join the ground for the input and output
capacitors and the return path for the LED current at the
exposed pad.
There are a few other aspects of the board design that
improve performance. An unbroken ground plane on
the second layer dissipates heat, but also reduces noise.
Likewise minimizing the area of the SW and BST nodes
reduces noise. The traces for FB and VC should be kept
short to lessen the susceptibility to noise of these highimpedance nodes. Matched Kelvin connections from theexternal current sense resistor to the ISP and ISN pins
are essential for current regulation accuracy. The 2.2μF
INTVCC and 1µF VREF capacitors as well as the 33nF BST
capacitor should be placed as closely as possible to theirrespective pins. Use bypass capacitors for the DC input
nodes such as VIN, CTRL, and PWM (for internal PWM) to
reduce noise. Keep the RT and RP nodes small and awayfrom noisy signals. Finally, a diode with anode connected
to ground and cathode to the drain of the PWMTG MOSFET can protect that device from overvoltage caused byexcessive inductance in the LED string. Please refer to thedemo board layout of the LT3922-1 for more information.

Rev. A

22

For more information www.analog.com

LT3922-1
TYPICAL APPLICATIONS
2MHz, 93% Efficient 10W (30V, 333mA) Boost LED DriverL1, 4.7µH
VIN
8V TO 27V

33nF
SW

VIN
1M

4.7µF

BST

VOUT

0.47µF

GND

EN/UVLO

0.47µF

365k

VOUT
OVLO

LT3922-1

59.0k

1M
FB

4.7µF
33.2k

GND

VREF
1µF

ISP

100k
ANALOG DIM
PWM DIM

300mΩ

CTRL
PWM

ISN

SYNC/SPRDINTVCC
100k

PWMTG
FAULT
SS RT

2.2µF
10nF

RP

M1

ISMON
VC

30V
333mA
LED

10k

45.3k
2MHz

1nF
39221 TA02a

Efficiency vs VIN
100

VLED = 30V
fSW = 2MHz95
EFFICIENCY (%)

100:1 External PWM Dimming

90
ILED
100mA/DIV

85
80

VPWM
2V/DIV

75
100% PWM DUTY CYCLE
70

6

9

12

15

18 21
VIN (V)

24

27

20µs/DIV

3039221 TA02b

INFINITE PERSISTENCE
VIN = 12V
fPWM = 100Hz

39221 TA02c

Rev. AFor more information www.analog.com

23

LT3922-1
TYPICAL APPLICATIONS
333mA Boost LED Driver Using Internal PWM and Analog CTRL Dimming
L1
4.7µH
VIN
8V TO 27V

33nF

SW

VIN

BST VOUT

1M

4.7µF

0.47µF

GND

EN/UVLO

0.47µF

365k

VOUT
OVLO

LT3922-1

59.0k

1M
FB

4.7µF
33.2k

VCTRL

GND

CTRL
VREF

100k

1µF
0.1µF
(OPTION)

PWM

ISP

97.6k

300mΩ

SYNC/SPRDISN

INTVCC
100k

PWMTG
FAULT
SS RT

2.2µF
10nF

RP
45.3k
2MHz

M1

ISMON
VC
332k
122Hz

10k
1nF

30V
333mA
LED

39221 TA03a

L1: WURTH 74437324047
M1: VISHAY Si2319CDS

128:1 Internal PWM Dimming

10:1 Internal PWM Dimming

ILED
100mA/DIV

ILED
100mA/DIV

10µs/DIV

39221 TA03b

200µs/DIVVCTRL = 2V
VIN = 12V
fPWM = 122Hz

39221 TA03c

VCTRL = 2V
VIN = 12V
fPWM = 122Hz

128:1 Internal PWM with 20:1 Analog CTRL Dimming

ILED
10mA/DIV

10:1 Internal PWM with 20:1 Analog CTRL Dimming

ILED
10mA/DIV

10µs/DIV

39221 TA03d

VCTRL = 0.3V
VIN = 12V
fPWM = 122Hz

200µs/DIV

39221 TA03eVCTRL = 0.3V
VIN = 12V
fPWM = 122Hz
Rev. A24

For more information www.analog.com

LT3922-1
TYPICAL APPLICATIONS
333mA Boost LED Driver Using External PWM Dimming and SSFM
L1
4.7µH
VIN
8V TO 27V

33nF

SW

VIN
1M

4.7µF

BST VOUT

0.47µF

GND

EN/UVLO

0.47µF

365k

VOUT
OVLO

LT3922-1

59.0k

1M
FB

4.7µF
33.2k

GND

VREF
CTRL
PWM

1µF

ISP
300mΩ

SSFM

SYNC/SPRDISN

INTVCC

PWMTG

100k
FAULT
SS RT

2.2µF
10nF

RP
45.3k
2MHz

M1

ISMON
VC

30V
333mA
LED

10k
1nF

39221 TA04a

L1: WURTH 74437324047
M1: VISHAY Si2319CDS

122Hz 10:1 External PWM Dimming withand without SSFM

ILED (SSFM)
200mA/DIVILED (NO SSFM)
200mA/DIV
2ms/DIV

39221 TA04b

Rev. AFor more information www.analog.com

25

LT3922-1
TYPICAL APPLICATIONS
Low EMI 400kHz, 96% Efficient 10W (30V, 333mA) Boost LED Driver with SSFM
Efficiency vs VIN

L1
22µH

33nF

VIN
8V TO
27V

10µF

0.1µF
0402

SW

VIN

FB1

4.7µF

100

BST VOUT

1M

0.47µF

365k

33µF

VOUT
OVLO

1M

LT3922-1

59.0k

FB

4.7µF
33.2k

1µF

90

85

GND

VREF

VLED = 30V
fSW = 400kHz

95

0.47µF

GND

EN/UVLOEFFICIENCY (%)

INPUT EMIFILTERWITHOUT EMI FILTERSWITH EMI FILTERS

ISP

100k

2.2µF 300mΩ

CTRL

ANALOG DIM
PWM DIM

80

ISN

PWM

6

8 10 12 14 16 18 20 22 24 26 28
VIN (V)39221 TA05b

SYNC/SPRDINTVCC
100k

PWMTG

FAULT
SS

2.2µF

RT

RP

0402
0.1µF

10k

249k
400kHz

10nF

M1

ISMON
VC

D1

FB2
OUTPUTEMI FILTER

1nF

L1: COILCRAFT XAL5050-223MEB
M1: VISHAY Si2319CDS
FB1: WURTH 742792040
FB2: WURTH 742792097
D1: NXP PMEG4010CEJ

30V
333mA
LED

39221 TA05a

50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15

Average Radiated EMI Performance (CISPR25)AMPLITUDE (dBµV/m)

AMPLITUDE (dBµV/m)

Peak Radiated EMI Performance (CISPR25)

CLASS 5 PEAK LIMIT
LT3922-1 400kHz f SW PEAK EMI
0

100

200

300

400
500
600
FREQUENCY (MHz)

700

800

900

1000

50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15

CLASS 5 AVERAGE LIMIT
LT3922-1 400kHz f SW AVERAGE EMI
010039221 TA05c

200

300

400
500
600
FREQUENCY (MHz)

700

800

900

1000

39221 TA05d

Rev. A26

For more information www.analog.com

LT3922-1
TYPICAL APPLICATIONS
500mA Boost LED Driver Using Pulse Duty Cycle CTRL Input

L1
6.8µH
VIN
8V TO 20V

33nF

SW

VIN
4.7µF

1M

BST VOUT

0.47µF

GND

EN/UVLO

0.47µF

348k

VOUT
OVLO

LT3922-1

84.5k

1M
FB

4.7µF
33.2k

1µF
3V, 10kHz PULSE

VREF

GND

PWM
CTRL

ISP
200mΩ

SYNC/SPRDISN

INTVCC

PWMTG

100k
FAULT
SS RT

2.2µF
10nF

RP
45.3k
2MHz

M1

ISMON
VC
10k
1nF

L1: WURTH 74437324068
M1: VISHAY Si2319CDSVCTRL Duty Cycle Stepped from 15% to 75%

39221 TA06a

VCTRL Duty Cycle Stepped from 75% to 15%

ILED
200mA/DIV

ILED
200mA/DIVVCTRL
2V/DIVVCTRL
2V/DIV
1ms/DIV

24V
500mA
LED

39221 TA06b

1ms/DIV

39221 TA06c

Rev. AFor more information www.analog.com

27

LT3922-1
TYPICAL APPLICATIONS
2MHz, 95% Efficient 15W (15V, 1A) Buck Mode LED Driver
L1
4.7µH

33nF

VIN
22V TO 36V
VIN
20V TO 36V

BST

VIN

SW

VOUT

1M

33µF

EN/UVLO

0.47µF

51.1k

VOUT

1µF

OVLO

249k

LT3922-1

34.8k

20k
20k

Q1
FB

VREF

1µF

0.47µF

GND

22µF

33.2k

100k

GND

ANALOG DIM

CTRL

PWM DIM

PWM

ISP
100mΩ

SYNC/SPRDISN

INTVCC
100k

PWMTG
FAULT
SS RT

2.2µF
L1: WURTH 74437324047
M1: VISHAY Si2319CDS
Q1: ZETEX FMMT591A

RP
45.3k
2MHz

100nF

M1

ISMON
VC

4.7nF

15V
1A
LED

39221 TA07a

Efficiency vs VIN
100

EFFICIENCY (%)

95
90
85
80
75
70

16

20

24

28
VIN (V)

32

36

40

39221 TA07b

Rev. A28

For more information www.analog.com

LT3922-1
TYPICAL APPLICATIONS
500mA, 5V to 12V Boost Converter with Accurate Input Current Limit

56mΩ

VIN
5V
2.2µF

L1
4.7µH

33nF

SW

VIN
499k

BST VOUT

0.47µF

GND

EN/UVLO100
95

0.47µF

121k

VOUT

150k

90

549k

LT3922-1

FB

10µF
60.4k

GND

VREF

ISN

100k

EFFICIENCY (%)

OVLO

1µF

Efficiency

VOUT
12V
500mA

85
80
75
70

ISP

ANALOG DIM

CTRL

PWM DIM

PWM

65

SYNC/SPRD

60

INTVCC

PWMTG

100k

2.2µF

L1: WURTH 744316470

100

200
300
ILOAD (mA)

400

500
39221 TA08b

ISMON

FAULT
SS RT

10nF

0

RP
45.3k
2MHz

VC
10k
1nF
39221 TA08a

Rev. AFor more information www.analog.com

29

LT3922-1
TYPICAL APPLICATIONS
Shorted LED Robust Boost LED Driver
L1
4.7µH
VIN
8V TO 18V

SW

VIN

BST VOUT

1M

4.7µF

Shorted LED Protection without RSS:
Hiccup Mode

33nF

GND

EN/UVLO226k

0.47µFVPWMTG
20V/DIV

0.47µFVSS
2V/DIV

VOUT
OVLO

LT3922-1

80.6k

1M
FB

VFAULTB
2V/DIV

4.7µF
33.2k

VREF

1µF

CTRL

PWM DIM

PWM

20ms/DIV

ISP

100k
ANALOG DIM

ILED
1A/DIV

GND

200mΩ
ISN

Shorted LED Protection with RSS:
Latchoff ModeSYNC/SPRDINTVCC
100k
RSS
100k
OPTIONAL

PWMTG
FAULT
SS RT

2.2µF 10nF
L1: WURTH 74437324047
M1: VISHAY Si2319CDS
D1: NXP PMEG4010CEJ

39221 TA09b

ISMON
VC

RP
45.3k
2MHz

M1 D1

332k
122Hz

10k
1nF

21V
500mA
LED

VPWMTG
20V/DIV
VSS
2V/DIV
VFAULTB
2V/DIV39221 TA09aILED
1A/DIV
100ms/DIV

39221 TA09c

Rev. A30

For more information www.analog.com

LT3922-1
TYPICAL APPLICATIONS
2MHz, 88% Efficient 6.25W (12.5V, 500mA) Buck-Boost Mode LED Driver
L1
4.7µH
VIN
6V TO
18V

C4, 33nF

SW

VIN

C2
10µF

EN/UVLO

C3
1µF

VOUT

C11
0.47µF

GND

OVLO

C10
0.47µFVOUT

LT3922-1

R3
249k

R4
R5
20k
20k

Q1
FB

VREF
C5
1µF

C9
10µF

R6
39.2k

GND
CTRL

ISP
R7
0.2Ω

SYNC/SPRD
PWM

ISN

INTVCC
R1
100k

PWMTG

SS

M1

ISMON

FAULT

C6
2.2µF

RT

RP

VC
R8
10k

R2
45.3k
2MHz

C7
10nF

C8
1nF

12.5V
500mA
LED

39221 TA10a

L1: WURTH 7443551470
M1: VISHAY Si2319DS
Q1: ZETEX FMMT591A

Efficiency vs VIN
100
95
90
EFFICIENCY (%)

C1
33µF

BST

85
80
75
70
65
60

4

6

8

10

12 14
VIN (V)

16

18

20

39221 TA10b

Rev. AFor more information www.analog.com

31

LT3922-1
PACKAGE DESCRIPTIONUFD Package
28-Lead Plastic QFN (4mm × 5mm)

(Reference LTC DWG # 05-08-1712 Rev C)

0.70 ±0.05

4.50 ±0.05
3.10 ±0.05
2.50 REF

2.65 ±0.05
3.65 ±0.05

PACKAGE OUTLINE

0.25 ±0.05
0.50 BSC
3.50 REF
4.10 ±0.05
5.50 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONSAPPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
4.00 ±0.10(2 SIDES)

R = 0.05
TYP

0.75 ±0.05

PIN 1 NOTCH
R = 0.20 OR 0.35
× 45° CHAMFER

2.50 REF
R = 0.115
TYP
27

28
0.40 ±0.10

PIN 1
TOP MARK(NOTE 6)1
2

5.00 ±0.10(2 SIDES)

3.50 REF
3.65 ±0.10
2.65 ±0.10(UFD28) QFN 0816 REV C

0.200 REF
0.00 – 0.05

0.25 ±0.05
0.50 BSC
BOTTOM VIEW—EXPOSED PAD

NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGHD-3).
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATIONON THE TOP AND BOTTOM OF PACKAGE

Rev. A32

For more information www.analog.com

LT3922-1
REVISION HISTORY
REV

DATE

DESCRIPTION

A

02/21

Updated LED current regulation to 2.5% accuracy

PAGE NUMBER
1

Added AEC-Q100 Qualification and automotive model order information

1, 2Changed BST pin capacitor to 33 nF

1, 10, 12, 21-29, 30, 32

Clarified Absolute Maximum Ratings

2

Clarified VREF Voltage conditions

3

Revised Electrical Characteristic values3, 4Rev. A

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license For
is granted
implication orotherwise under any patent or patent rights of Analog Devices.
morebyinformationwww.analog.com

33

LT3922-1
TYPICAL APPLICATION
Low EMI 2MHz, 333mA Boost LED Driver with SSFM
L1
4.7µH

0.1µF
0402

SW

VIN

FB1
33µF

4.7µF

BST VOUT

1M

0.47µF

GND

EN/UVLOAMPLITUDE (dBµV/m)

VIN
8V TO
27V

Peak Radiated EMI Performance (CISPR25)

33nF

INPUT EMIFILTER

0.47µF

365k

VOUT
OVLO

1M

LT3922-1

59.0k

FB

2.2µF
33.2k

GND

VREF

1µF

ISP

100k
ANALOG DIM

CTRL

PWM DIM

PWM

300mΩ

2.2µF

50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15

ISN

CLASS 5 PEAK LIMIT
LT3922-1 2MHz f SW PEAK EMI
0

100

200

300

SYNC/SPRD

400
500
600
FREQUENCY (MHz)

700

800

900

1000

39221 TA11b

INTVCC

SS

2.2µF

M1

ISMON

FAULT
RT

100nF

RP
45.3k
2MHz

VC
332k
122Hz

24k

0402
0.1µF

D1

0.22nF

L1: WURTH 74437324047
M1: VISHAY Si2319CDS
FB1: WURTH 7427920415
FB2: WURTH 742792097
D1: NXP PMEG4010 CEJ

Average Radiated EMI Performance (CISPR25)FB2OUTPUTEMI FILTER

30V
333mA
LED

39221 TA11a

AMPLITUDE (dBµV/m)

PWMTG

100k

50
45
40
35
30
25
20
15
10
5
0
–5
–10
–15

CLASS 5 AVERAGE LIMIT
LT3922-1 2MHz f SW AVERAGE EMI
0

100

200

300

400
500
600
FREQUENCY (MHz)

700

800

900

1000

39221 TA11c

RELATED PARTS
PART NUMBER

DESCRIPTIONCOMMENTS

LT3922

36V, 2A Synchronous Step-Up LED Driver

VIN = 2.8V to 36V, VOUT(MAX) = 40V, 128:1 Internal Dimming and 5,000:1 External
Dimming, ISD = 1µA, 4mm × 5mm QFN-28

LT3932/LT3932-1 36V, 2A Synchronous Step-Down LED Driver

VIN = 3.6V to 36V, VOUT = 0V to 36V, 128:1 Internal Dimming and 5,000:1/10,000+:1
External Dimming, ISD = 1µA, 4mm × 5mm QFN-28

LT3952

60V, 4A LED Driver with 4000:1 PWM Dimming with VIN: 3V to 42V, VOUT(MAX) = 60V, 4000:1 PWM, 20:1 Analog, ISD < 1µA,
TSSOP-28E Package
Spread Spectrum

LT3518

2.3A, 2.5MHz High Current LED Driver with 3000:1 VIN: 3V to 30V, VOUT(MAX) = 45V, 3000:1 PWM Dimming, ISD < 1μA,
4mm × 4mm QFN-16 and TSSOP-16E Packages
Dimming with PMOS Disconnect FET Driver

LT3755/LT3755-1/ 40VIN, 75VOUT, 1MHz Non-Synchronous Boost LED VIN: 4.5V to 40V, VOUT: VIN to 75V, ±4% Current Accuracy, 3mm × 3mm
Controller
QFN-16 and MSE-16
LT3755-2
LT376160VIN, 80VOUT, 1MHz Non-Synchronous Boost LED VIN: 4.5V to 60V, VOUT: VIN to 80V, ±3% Current Accuracy, External and Internal
Controller with Internal PWM Generator
PWM Dimming, MSE-16

LT3763

60V, 1MHz Synchronous Buck LED Controller

LT3795

110V, 1MHz Non-Synchronous Boost LED Controller VIN: 4.5V to 110V, VOUT: VIN to 110V, ±3% Current Accuracy, Internal
Spread Spectrum, TSSOP-28
with Spread Spectrum Frequency Modulation

LT8391

60V, 650kHz Synchronous 4-Switch Buck-Boost LED VIN: 4V to 60V, VOUT: 0V to 60V, ±3% Current Accuracy, External and Internal
PWM Dimming, Spread Spectrum, TSSOP-28 and 4mm × 5mm QFN-28
Controller with Spread Spectrum

VIN: 6V to 60V, VOUT: 0V to VIN –2V, ±6% Current Accuracy, TSSOP-28

Rev. A34

02/21

www.analog.com

For more information www.analog.com

 ANALOG DEVICES, INC. 2018–2021