LTC3490
Single Cell 350mA
LED DriverUFEATURESDESCRIPTIO

350mA Constant Current Output■ 2.8V to 4V Output Compliance■ 1- or 2-Cell NiMH or Alkaline Input■ Synchronous Rectification: Up to 90% Efficiency■ Fixed Frequency Operation: 1.3MHz■ Low Quiescent Current: <1mA
■ Very Low Shutdown Current: <50µA■ Open LED Output Limited to 4.7V
■ V Range: 1V to 3.2VIN■ Dimming Control■ Undervoltage Lockout to Protect Batteries■ Low Profile (0.75mm) 3mm × 3mm Thermally
Enhanced 8-Lead DD and S8 Packages

The LTC®3490 provides a constant current drive for 1W
LED applications. It is a high efficiency boost converter
that operates from 1 or 2 NiMH or alkaline cells andgenerates 350mA of constant current with up to 4V ofcompliance. It contains a 100mΩ NFET switch and a130mΩ PFET synchronous rectifier. The fixed switchingfrequency is internally set to 1.3MHz.

■

UAPPLICATIO S■■

Portable Lighting
Rechargeable Flashlights

The LTC3490 limits the output voltage to 4.7V if the output
load is disconnected. It also features an analog dimming
capability that reduces the drive current proportional tothe CTRL/SHDN pin voltage. A low-battery logic output
signals when the battery has dropped below 1V/cell. Anundervoltage lockout circuit shuts down the LTC3490
when the battery voltage drops below 0.85V/cell. The
feedback loop is internally compensated to minimize component count.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.

UTYPICAL APPLICATIO

Efficiency vs VIN at VLED = 3.5VSingle Cell Minimum Component LED Driver
100

3.3µH

IOUT = 350mA

9080SW

LTC3490

1 NiMH OR +ALKALINECELL

CTRL/SHDN LED
CELLS
1M

70CAP

ON/OFF

LOBAT
GND

350mA
4.7µF
HIGH
CURRENTLEDEFFICIENCY (%)

VIN

6050403020103490 TA01

011.52
VIN (V)

2.5

33490 TA023490fa

1

LTC3490

W W

W

AXI U

U

ABSOLUTERATI GS (Note 1)

Supply Voltage (VIN) ................................... – 0.3V to 6V
Input Voltages (CTRL/SHDN, CELLS) ......... – 0.3V to 6V
Output Voltages (CAP, LED, SW) ................ – 0.3V to 6V

Operating Temperature Range (Note 2) .. – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 125°C
Lead Temperature (Soldering, 10 sec, S8) .......... 300°CUW

U

PACKAGE/ORDER I FOR ATIOTOP VIEWTOP VIEWCELLS 18CTRL/SHDN

VIN 27LOBAT6CAP5LEDSW 39GND 4CELLS 18CTRL/SHDN

VIN 27LOBATSW 36CAP

GND 45LEDDD PACKAGE
8-LEAD (3mm × 3mm) PLASTIC DFN

S8 PACKAGE
8-LEAD PLASTIC SO

TJMAX = 125°C, θJA = 43°C/ W (NOTE 4)EXPOSED PAD (PIN 9) IS GND
MUST BE SOLDERED TO PCB (NOTE 5)

TJMAX = 125°C, θJA = 150°C/ W (NOTE 4)

ORDER PART NUMBER

DD PART MARKINGORDER PART NUMBER

S8 PART MARKING

LTC3490EDD

LBRQ

LTC3490ES83490

Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/Consult LTC Marketing for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. VIN = 2.5V unless otherwise specified.
SYMBOLPARAMETER

VIN

Input Supply Range

CONDITIONSVIN(START)

Minimum Start-Up Voltage(Note 3)

ILED(MAX)

LED Drive Current

VCTRL/SHDN = VIN, DD Package
25°C to 85°C
– 40°C to <25°C
VCTRL/SHDN = VIN, S8 Package
25°C to 85°C
– 40°C to <25°C

MIN

TYP

MAX

UNITS

3.2

V

0.9

1

V

330
310

350
350

370
385

mA
mA

337
325

350
345

363
365

mAmA

0.1

1µA

4V

1ILED(SHDN)

LED Drive Current in Shutdown

VLED

Output Compliance Voltage

VCTRL/SHDN = 0V

VLED(OVL)

Output Voltage Overvoltage Limit

Open LED

4.7VIIN(SHDN)

Input Current, Shutdown

VCTRL/SHDN = 0V, Excluding Switch Leakage

2050µA

IIN(ACTIVE)

Input Current, Active

Excluding Load Power

2030mAfSW

Switching Frequency

1.3

1.6

MHzIL(NMOS)

Leakage Current, NMOS Switch

0.1

RON(NMOS)

On-Resistance, NMOS Switch

0.1

●

2.8

●

4.2

●

1.0µA
Ω
3490fa2

LTC3490

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C. VIN = 2.5V unless otherwise specified.
SYMBOL
IL(PMOS)PARAMETER
Leakage Current, PMOS Switch

RON(PMOS)

On-Resistance, PMOS Switch

VIH

Input High (CELLS)CONDITIONS

MIN

MAX

UNITS
µA
Ω0.13
VIN – 0.4

Input High (SHDN)
VIL

TYP
0.1

V
VIN • 0.9

Input Low (CELLS)

V
0.4

Input Low (SHDN)V

VIN • 0.2

V

0.01

µA

500

mA/V

IIN

Input Current (CTRL/SHDN, CELLS)KCTRLControl Gain, ILED/VCTRLRON(LOBAT)

On-Resistance, LOBAT Output

VIN < VIN(LOBAT)●300Ω

VIN(LOBAT1)

Input Voltage, Low Battery, 1 Cell

VCELLS = 0V

●

0.8

1.12

VVIN(LOBAT2)

Input Voltage, Low Battery, 2 Cells

VCELLS = VIN

●

1.8

2.24

VVIN(UVLO2)

Input Voltage, Undervoltage Lockout, VCELLS = VIN
2 Cells

●

1.4

1.8

VVIN(UVLO1)

Input Voltage, Undervoltage Lockout, VCELLS = 0 V
1 Cell

●

0.7

0.9

V

Scales Linearity with VIN, VIN = 1V

Note 4: This device includes overtemperature protection intended toprotect the device during momentary overload conditions. The maximum
junction temperature may be exceeded when overtemperature protectionis active. Continuous operation above the specified maximum operating
junction temperature may result in device degradation or failure.
Note 5: The Exposed Pad of the DFN package must be soldered to a
PCB pad for optimum thermal conductivity. This pad must be connected
to ground.

Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any AbsoluteMaximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC3490 is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C range areassured by design, characterization and correlation with statistical processcontrols.
Note 3: The LTC3490 input voltage may drop below the minimum start-up
voltage once the LED voltage has risen above 2.3V.

U W

TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
vs Temperature

ILED vs VCTRL

1.400

ILED vs VIN
375

400

VLED = 3.5V

350
350

1.360

1.280

250

ILED (mA)

1.320

ILED (mA)

FREQUENCY (MHz)300

MAXIMUM200150325

300

MINIMUM
1002751.240
501.200–50500
TEMPERATURE (°C)

1003490 G01

0250
00.20.40.6
VCTRL/VIN (V)0.8

13490 G02

11.52
VIN (V)

2.5

33490 G03

3490fa3

LTC3490
U W

TYPICAL PERFOR A CE CHARACTERISTICS
ILED vs VLED
360

Efficiency vs ILED100

VIN = 2.4V

35835680

EFFICIENCY (%)

354

ILED (mA)

VIN = 2.4V

90352
350
348

5040346303442034210340

VIN = 1.2V

70
60

032.83.2

3.4
3.6
VLED (V)3.843490 G04

0100200
ILED (mA)

4003003490 G05

UUU

PI FU CTIO SCELLS (Pin 1): A logic input to set the low-battery andundervoltage shutdown thresholds. A logic low (tied toGND) will set the thresholds for 1 cell. A logic high (tied toVIN) will set the thresholds for 2 cells.
VIN (Pin 2): Supply Voltage.
SW (Pin 3): Switch Input. Connect this pin to an external
inductor from VIN.
GND (Pin 4): Circuit Ground.
LED (Pin 5): Output Drive Current to LED.
CAP (Pin 6): Filter Capacitor. A 4.7µF low ESR capacitor
should be tied to this pin.

CTRL/SHDN (Pin 8): Analog Control Voltage and Shutdown. When VIN • 0.2 < VCTRL < VIN • 0.9, the LED drive
current varies according to the formula:⎛V⎞
ILED = 500 • ⎜ CTRL – 0.2⎟ mA⎝ VIN⎠

When VCTRL > VIN • 0.9, the LED drive current is clamped
at 350mA. When VCTRL < VIN • 0.2, then the part is in lowpower shutdown.
Exposed Pad (Pin 9, DD Package): Ground. This pin must
be soldered to the PCB to provide both electrical contact
to ground and good thermal contact to the PCB.

LOBAT (Pin 7): Low active, open-drain logic output indicating a low-battery condition.

3490fa4

LTC3490W
FU CTIO AL DIAGRA
U

3SW–

2P BODY
CONTROL+VIN

CAP

6GATE
CONTROL
AND
DRIVERS
SENSE
AMP

LIMIT

19.2Ω

+–

OVERVOLTAGE
DETECT

LED

5250k–

PWM
LOGIC

0.1Ω

40k
VREF/2+OSCILLATORSTART-UP–

8CTRL/SHDNDIMMINGAMP+LOBAT

IREF

1BATTERYMONITORCELLS

7SHUTDOWN

GND
43490 FD

3490fa5U

LTC3490

UOPERATIOThe LTC3490 is a high efficiency, constant current sourcefor 1W high intensity white LEDs. These high intensity
LEDs require a fixed current of 350mA with a voltage
compliance of 2.8V to 4V.
The LTC3490 operates with 1 or 2 NiMH or alkaline cells.
It functions as a boost converter with a current sense resistor providing the control feedback. If the battery voltage
is greater than the required LED compliance, it will cycle
off periodically to maintain the correct average current. Itfeatures a low voltage start-up circuit that will start with aninput voltage of only 1V. Once the drive voltage exceeds
2.3V, the circuit operates from the drive voltage.

to the battery voltage. The LED drive current is given by theformula:⎛V⎞
ILED = 500 • ⎜ CTRL – 0.2⎟ mA⎝ VIN⎠
When VCTRL > VIN • 0.9, the LED drive current is clamped
at 350mA.
Open-Circuit Protection

All of the loop compensation is internal; only the main filter
capacitor is needed for stable operation.

Since this is a boost converter attempting to drive a current into the load, an open or high impedance load will cause
the regulator loop to increase the output voltage in an effort to achieve regulation. To protect the device, maximum
output voltage is limited to 4.7V under all conditions.

Dimming Function

Undervoltage Sense and Protection

During normal operation with the CTRL/SHDN pin connected to VIN, the LED drive current is controlled at350mA. The drive current can be reduced by changing thevoltage on the CTRL/SHDN pin.

The undervoltage lockout prevents excessive inductor
peak current and protects the batteries from deep discharging which can damage them. The low-battery indicator allows the end user to be made aware that the batteries
are nearing the end of their useful life.

For VIN • 0.2 < VCTRL < VIN • 0.9, the LED current is
proportional to VCTRL/VIN. This allows a simple potentiometer from VIN to control the current without sensitivity

3490fa6

LTC3490UWU U

APPLICATIO S I FOR ATIOThe LTC3490 requires only four external components tooperate: an inductor, an output capacitor, a switch and apull-down resistor. The inductor is nominally set at 3.3µH
and the capacitor at 4.7µF. Optional components include
an input capacitor and dimming resistors.

where:
VIN = Input Voltage (V)VOUT = Output Voltage (V)
IOUT = LED Drive Current (A)IIN = Input Current = VOUT/VIN • IOUT (A)COMPONENT SELECTION

RP = RDSON of the PFET Switch (Ω)
Inductor Selection

RN = RDSON of the NFET Switch (Ω)

The high frequency operation of the LTC3490 allows theuse of small surface mount inductors. The minimum
inductance value is proportional to the operating frequency and is limited by the following constraints:
3L≥ HfandL≥(VIN(MIN) • VOUT(MAX) – VIN(MIN)f • Ripple • VOUT(MAX))H

where:
f = Operating Frequency (Hz)Ripple = Inductor Current Ripple (A)VIN(MIN) = Minimum Input Voltage (V)VOUT(MAX) = Maximum Output Voltage (V)The inductor current ripple is typically set to 20% to 40%of the inductor current.
The peak inductor current is given by:
ILPK = IOUT

For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce corelosses. The inductor should have low ESR (equivalentseries resistance) to reduce the I2R losses and must beable to handle the peak inductor current at full load without
saturating. In single cell applications, the inductor ESR
must be below 25mΩ to keep the efficiency up andmaintain output current regulation. Dual cell applications
can tolerate significantly higher ESR (up to 75mΩ) withminimal efficiency degradation. Molded chokes or chip
inductors usually do not have enough core to support thepeak inductor currents in the 1A to 2A region. If radiatednoise is an issue, use a toroid, pot core or shielded bobbininductor to minimize radiated noise. See Table 1 for a listof suggested inductors. Look closely at the manufacturers
data sheets; they specify saturation current differently.
Table 1. Inductor Information
INDUCTOR PART NUMBER

ESR (mΩ)

SATURATION CURRENT (A)TOKO A918CY-3R3M

47

1.97

TYCO DN4835-3R3M

58

2.15

TDK SLF7045T-3R3M2R5202.5( VOUT + IOUT • RP ) – RN • IIN

Output Capacitor Selection

VIN – RN • IIN
VIN ( VOUT – VIN )+2 • L • f • VOUT

The output capacitor value and equivalent series resistance (ESR) are the primary factors in the output ripple.
The output ripple is not a direct concern for LED drive as
the LED will operate at the average current value. However
the peak pulsed forward current rating of the LED must not
be exceeded to avoid damaging the LED.

3490fa7

LTC3490UWU U

APPLICATIO S I FOR ATIOThe output ripple voltage has two primary components.
The first is due to the value of the capacitor and is given by:

VRCAP =ILPK • VINC • VOUT • f

The second is due to the capacitor ESR:VRESR = ILPK • RESR

Component values will be calculated for 1 or 2 NiMH cells
and assumes the end-of-charge voltage to be 0.9V per cell.
The operating frequency is assumed to be 1MHz, theworst-case low frequency. The allowed inductor ripple
current is 0.31A. Table 3 shows a summary of the key
parameters.
Table 3. Summary of Key Parameters
PARAMETER

The LED current ripple and peak pulsed current are calculated by:
VRCAP • VRESRR SENSE + RLED
IRIPPFC = IOUT + LE D
2IRLED =1-CELL2-CELLUNITSLMIN

2.2

3.2

µH

Choose L

3.3

3.3

µH

IIN

1.56

0.78

A

ILPK

1.93

0.96

A

Choose C

4.7

4.7

µF

Cap ESR

55mΩ

VRCAP

0.09

0.09

V

VRESR

0.01

0.005

VIRLED

0.10

0.09

A

RSENSE = Internal Sense Resistor = 0.1Ω

IPPFC

0.40

0.39

A

RLED = Dynamic Impedance of the LED

where:where:

Low ESR capacitors should be used to minimize output
ripple. Ceramic X5R or X7R type capacitors are recommended. See Table 2 for a list of component suppliers.
Table 2. Capacitor Information
CAPACITOR PART NUMBER

DESCRIPTIONTDK C2012X5R0J475K

4.7µF, 6.3V, X5R in 0805

AVX 1210ZC475MAT

4.7µF, 10V, X7R in 1210

Taiyo Yuden CELMK316BJ475ML

4.7µF, 10V, X7R in 1206

ILPK is the peak inductor current
VRCAP is the ripple voltage due to the output capacitor
value
VRESR is the ripple voltage due to the output capacitor
ESR
IRLED is the LED current ripple
IPPFC is the LED peak pulsed forward current
PC Board Layout Checklist

Input Capacitor Selection
Most battery-powered applications do not need an input
capacitor. In supply-powered applications or battery applications with long leads to the battery, a low ESR 3.3µF
capacitor reduces switching noise and peak currents.
Design Example
The example will use a Lumileds DS25 white LED. The key
specifications are:

Keep the inductor and output capacitor as close to the IC
as possible. Make traces as short and wide as is feasible.
Parasitic resistance and inductance reduce efficiency andincrease ripple.
Keep resistance in the battery connections as low as
possible. In single cell applications, only 0.1Ω in thebattery connections will have a dramatic effect in efficiency and battery life. I2R losses can exceed 100mW andthe converter operates lower on the efficiency curve.

VF (at IF= 350mA) = 3.4 ±0.6V
RLED = 1Ω
Peak Pulsed Forward Current = 0.5A
3490fa8

LTC3490UWU U

APPLICATIO S I FOR ATIORed Luxeon LEDs

is only 2.31V. The LTC3490 requires an additional 190mVfor proper operation. In non-dimming applications, this canbe accomplished with a 0.6Ω resistor in series with theLED. The resistor voltage drops too low in dimming applications, so a Schottky diode is recommended to keep
sufficient voltage at the output at lower currents.

The red, red-orange and amber Luxeon LEDs have a lower
forward voltage than the white, blue and green LEDs. Since
the LTC3490 internal circuitry is powered from the output,
it requires a minimum LED voltage of 2.5V for reliable
operation. The minimum forward voltage on the red LEDsU

TYPICAL APPLICATIO S
2-Cell Adjustable Amplitude LED Driver
3.3µH

VINSW

ON/OFFCAP+

2 NiMH ORALKALINECELLS +

LTC3490
4.7µF

CTRL/SHDN LED
LOBAT

CELLS
1M

LUMILEDSLUXEON
LXHL-BW02

GND

3490 TA03

Soft Turn-Off LED Driver
3.3µH

VIN

SW
CAP

ON/OFF
LTC3490

1 NiMH OR +ALKALINECELL

CTRL/SHDN LED
CELLS
1µF

1M

LOBAT
GND

350mA
4.7µF
LUMILEDSLUXEON
LXHL-BW02

3490 TA04

3490fa9

LTC3490

U

TYPICAL APPLICATIO S
Luxeon Red LED Driver Without Dimming
3.3µH

ON/OFFSW

VIN

CAP
LTC3490

1 NiMH OR +ALKALINE
CELL

4.7µF

CTRL/SHDN LED
0.6Ω

LOBAT

CELLS
1M

LUMILEDSLUXEON
LXHL-BD03

GND

3490 TA06

Luxeon Red LED Driver with Dimming
3.3µH

ON/OFFSW

VIN

CAP
1 NiMH OR +ALKALINE
CELL

LTC3490
4.7µF

CTRL/SHDN LED

1M

MBRM120E

LOBAT

CELLS

LUMILEDSLUXEON
LXHL-BD03

GND

3490 TA07

Efficiency vs VIN with Red LED
90
RESISTOR80

SCHOTTKY

EFFICIENCY (%)

70
60
50
40
30
20
10011.5

22.5

3

VIN (V)3490 G06

3490fa10

LTC3490

U

PACKAGE DESCRIPTIODD Package
8-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1698)
R = 0.115
TYP
50.38 ± 0.10
80.675 ±0.05

3.5 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)

3.00 ±0.10(4 SIDES)
PACKAGEOUTLINE

1.65 ± 0.10
(2 SIDES)PIN 1TOP MARK(NOTE 6)(DD8) DFN 1203

0.25 ± 0.05

40.25 ± 0.05

0.75 ±0.05

0.200 REF
0.50
BSC
2.38 ±0.05(2 SIDES)

10.50 BSC

2.38 ±0.10(2 SIDES)

0.00 – 0.05

BOTTOM VIEW—EXPOSED PADRECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-1)
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 TOP AND BOTTOM OF PACKAGE

S8 Package
8-Lead Plastic Small Outline (Narrow .150 Inch)(Reference LTC DWG # 05-08-1610).189 – .197
(4.801 – 5.004)
NOTE 3.045 ±.005.050 BSC8.245MIN

765.160 ±.005.150 – .157(3.810 – 3.988)
NOTE 3.228 – .244
(5.791 – 6.197).030 ±.005
TYP

1RECOMMENDED SOLDER PAD LAYOUT.010 – .020
× 45°
(0.254 – 0.508).008 – .010
(0.203 – 0.254)

34.053 – .069
(1.346 – 1.752).004 – .010
(0.101 – 0.254)

0°– 8° TYP.016 – .050
(0.406 – 1.270)
NOTE:
1. DIMENSIONS IN

2.014 – .019
(0.355 – 0.483)
TYP

INCHES(MILLIMETERS)
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm).050(1.270)
BSC

SO8 0303

3490faInformation furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

11LTC3490

U

TYPICAL APPLICATIO

LED Driver Drops to 20% Amplitude on Low-Battery Detect
3.3µH

ON/OFFVINSW
CAP

1 NiMH OR +ALKALINE
CELL

LTC3490

1M

350mA/70mA

CTRL/SHDN LED
CELLS
1M

432k

LOBAT
GND

4.7µF
LUMILEDSLUXEON
LXHL-BWO23490 TA05

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3490fa12Linear Technology Corporation

LT 0606 REV A • PRINTED IN THE USA

1630 McCarthy Blvd., Milpitas, CA 95035-7417(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com© LINEAR TECHNOLOGY CORPORATION 2005