L6920
1V HIGH EFFICIENCY SYNCRONOUS STEP UP CONVERTER
1Figure 1. PackageFeatures■ 0.6 TO 5.5V OPERATING INPUT VOLTAGE■ 1V START UP INPUT VOLTAGE■ INTERNAL SYNCHRONOUS RECTIFIERTSSOP8■ ZERO SHUT DOWN CURRENT

5V FIXED OR ADJUSTABLEOUTPUT VOLTAGE (2V UP TO 5.2V)■ 120mΩ INTERNAL ACTIVE SWITCH■ LOW BATTERY VOLTAGE DETECTION■ REVERSE BATTERY PROTECTION

Table 1. Order Codes

1.1 Applications

2■ 3.3V AND■ ONE TO THREE CELL BATTERY DEVICES■ PDA AND HAND HELD INSTRUMENTS■ CELLULAR PHONES - DIGITAL CORDLESSPHONEPart NumberPackage

L6920DTSSOP8 Tube

L6920DTR

Tape & ReelDescription

The L6920 is a high efficiency step-up controller requiring only three external components to realize theconversion from the battery voltage to the selected
output voltage.
The start up is guaranteed at 1V and the device is operating down to 0.6V.

■ PAGERS■ GPSInternal synchronous rectifier is implemented with a120mΩ P-channel MOSFET and, in order to improvethe efficiency, a variable frequency control is implemented.

■ DIGITAL CAMERAS

Figure 1. Application Circuit

L1VCC
2.5V

February 2005

LX

87

C21SHDN

5LBI

23REF

46L6920D

OUT

VOUT
3.3V
500mA

FB
C3C1LBO
GND

Rev. 21/13

L6920
Table 1. Pin Description
PinNameFunction

1FBOutput voltage selector. Connect FB to GND for Vout=5V or to OUT for Vout=3.3V. Connect FB to an
external resistor divider for adjustable output voltage (from 2V to 5.2V) [see R4 and R5, fig. 7].

2LBI

Battery low voltage detector input. The internal threshold is set to 1.23V.
A resistor divider is needed to adjust the desired low battery threshold:
R1V LBI = 1.23V ⋅ ⎛ 1 + --------⎞ [see R1 and R2, fig. 7]
⎝R2⎠

3LBO

Battery low voltage detector output. If the voltage at the LBI pin drops below the internal
threshold typ. 1.23V, LBO goes low.
The LBO is an open drain output and so a pull-up resistor (about 200KΩ) has to be added forcorrect output setting [see R3, fig. 7].

4REF

1.23V reference voltage. Bypass this output to GND with a 100nF capacitor for filtering highfrequency noise. No capacitor is required for stability

5SHDN Shutdown pin. When pin 5 is below 0.2V the device is in shutdown, when pin 5 is above 0.6V thedevice is operating.

6GND

Ground pin

7LX

Step-up inductor connection

8OUT

Power OUTPUT pin

Figure 2. Pin Connection (Top view)

FB

18OUT

LBI

27

LX

LBO

36

GND

REF

45

SHDN

TSSOP8

Table 2. Absolute Maximum Ratings
SymbolVccmax

Vout max

Parameter

Value

Unit

Vcc to GND

6V

LBI, SHDN, FB to GND

6V

Vout to GND

6V

Value

UnitThermal Resistance Junction to Ambient

250°C/W

Maximum Junction Temperature

150°C

Table 3. Thermal Data
SymbolRth j-amb
Tj

2/13ParameterL6920
Table 4. Electrical Characteristcs(Vin = 2V, FB = GND, Tamb = -40°C to 85°C and Tj < 125°C unless otherwise specified)SymbolParameter

Test Condition

Min.

Typ.

Max.

Unit

VCC SECTION
Vin

Minimum operating Input Voltage

Vin

Minimum Start Up Input Voltage

Iq

Quiescent Current

0.6

V1V

Il =0 mA, FB = 1.4V, Vout = 3.3V
LBI = SHDN = 2V, Tj = Tamb915µA

Il =0 mA, FB = 1.4V, Vout = 5V
LBI = SHDN = 2V, Tj = Tamb

1118µA

IsdShut Down Current

Vin = 5V, Il =0 mA

0.1

1µA

Irev

Reverse battery current

Vin = -4V, Tj = Tamb

0.1

2µA

POWER SECTION
Ron-N

Active switch ON resistance

120

250mΩ

Ron-P

Synchronous switch ONresistance

120250mΩ

CONTROL SECTION
Vout

Output voltage

Output voltage range
VLBI

FB = OUT, Il =0 mA

3.2

3.3

3.4

V

FB = GND, Il =0 mA

4.955.1

V

External divider25.2

V

LBI threshold
0°C < Tj < 70°C

VLBO

LBO logic LOWIlim

LX switch current limit

1.18

1.23

1.27

V

1.205

1.23

1.255

V

0.2

0.4

V

0.8

11.2

A

Isink < 250µA

TonmaxMaximum on time

Vout = 2V to 5.3V

3.75

56.25

µs

Toffmin

Minimum off time

Vout = 2V to 5.3V

0.75

11.25µsSHDN

SHDN logic LOW

0.2

V

Vref

SHDN logic HIGH

0.6

Reference Voltage

1.18

V
1.23

1.27

V

3/13

L6920Figure 3. Efficiency vs. Output Current

Figure 5. Startup Voltage vs Output Current
1.4100Vin = 2.4V
901.280

Vin = 1.2V
1Startup voltage (V)EFFICIENCYη [%]7060504030Vout = 3.3V
L = 47µH
C = 100µF

200.80.60.410

L = 47µH
C = 22µF

0.200.010.1

1101001000

LOAD CURRENT [mA]

030100Vin = 3.6V90Vin = 2.4V80

Vin = 1.2V

EFFICIENCYη [%]706050fVout= 5V
L = 47µH
C = 100µF

4030201000.010.1110LOAD CURRENT [mA]

4/131006090120Output current (mA)

Figure 4. Efficiency vs. Output Current

1000150180

L69203Detailed Description

The L6920 is a high efficiency, low voltage step-up DC/DC converter particularly suitable for 1 to 3 cells (Li-Ion/polymer, NiMH respectively) battery up conversion.
These performances are achieved via a strong reduction of quiescent current (10µA only) and adopting a synchronous rectification, that implies also a reduced cost in the application (no external diode required).
Operation is based on maximum ON time - minimum OFF time control, tailored by a current limit set to 1A. A
simplified block diagram is shown here below.
Figure 6. Simplified Block Diagram

VOUT

OUT
ZERO CROSSING
-VREF

++-+

VBG

SHDN

A

FB

Y

VOUT
GND
R1,R2A
B
CYB

-VOUT
LX

OPAMP
(CR)

+C

VBG
-Q

Toff min
1µsec

S+FB

Ton max
5µsec

VBG

LBI

4GND

+

R

CURRENT LIMIT
LBO

VIN

D99IN1041

Principle of Operation

In L6920 the control is based on a comparator that continuously checks the status of output voltage.
If the output voltage is lower than the expected value, the control function of the L6920 directs the energy stored
in the inductor to be transferred to the load. This is accomplished by alternating between two basic steps:- TON phase: the energy is transferred from the battery to the inductor by shorting LX node to ground via the Nchannel power switch. The switch is turned off if the current flowing in the inductor reaches 1A or after a maximum on time set to 5µs.
- TOFF phase: the energy stored in the inductor is transferred to the load through the synchronous switch for atleast a minimum off time equal to 1µs. After this, the synchronous switch is turned off as soon as the output
voltage goes lower than the regulated voltage or the current flowing in the inductor goes down to zero.
So, in case of light load, the device works in PFM mode, as shown in figures 7 to 10.

5/13

L6920Figure 7. PFM mode Condition: Vout = 5V; Vin =1.5V.
Trace1: Vout (50mV~/div) Trace 4: IL (100mA/div)
Time div.: 5µs/div

Figure 9. Heavy load - Inductor current ripples
below Ilim Trace1: Vout (100mV~/div) Trace 4: IL(200mA/div) Time div.: 20 µs/div

Figure 8. Heavier load - Train pulses overlapping.
Trace1: Vout (100mV~/div) Trace 4: IL (200mA/div)
Time div.: 10 µs/div

Figure 10. Heavy load and High ESR. Regulation
falls in continuous mode of operation. Trace1:Vout (100mV~/div) Trace 4: IL (200mA/div). Time
div.: 5 µs/div

When Iload is heavier, the pulse trains are overlapped. Figures 7 - 8 show some possible behaviors.
Considering that current in the inductor is limited to 1A, the maximum load current is defined by the followingrelationship:V in
V out – V in
I load_lim = ----------- ⋅ ⎛ I lim – T off min ⋅ --------------------------⎞ ⋅ η eq. (1)2⋅L ⎠V out ⎝Where η is the efficiency and Ilim =1A.
Of course, if Iload is greater than Iload_lim the regulation is lost (figure 11).

6/13

L6920Figure 11. No regulation. Iload > Iload_lim
Trace1: Vout (100mV~/div) Trace 4: IL (200mA/div).
Time div.: 5 µs/div

The synchronous switch body diode causes a parasitic path between power supply and output that can'tbe avoided also in shutdown.
4.3 Low battery detection
The L6920 includes a low battery detector comparator. Threshold is VREF voltage and a 1.3% hysteresis is added to avoid oscillations when input crosses
the threshold slowly. The LBO is an open drain output so a pull up resistor is required for a proper use.
4.4 Reverse polarity
A protection circuit has been implemented to avoid
that L6920 and the battery are destroyed in case ofwrong battery insertion.

4.1 Start-up
One of the key features of L6920 is the startup at supply voltage down to 1V (please see the diagram inFigure 5. in case of heavy load).
The device leaves the startup mode of operation assoon as VOUT goes over 1.4V. During startup, thesynchronous switch is off and the energy is transferred to the load through its intrinsic body diode.
The N-channel switches with a very low RDSon
thanks to an internal charge pump used to bias thepower mos gate. Because of this modified behavior,
TON/TOFF times are lengthened. Current limit andzero crossing detection are still available.
4.2 Shutdown
In shutdown mode (SHDN pulled low) all internal circuitries are turned off, minimizing the current provided by the battery (ISHDN < 100 nA, in typical case).
Both switches are turned off, and the low battery
comparator output is forced in high impedance state.

In addition, this circuit has been designed so that thecurrent required by the battery is zero also in reverse
polarity.

5Application Information

5.1 Output voltage selection
Output voltage must be selected acting on FB pin.
Three choices are available: fixed 3.3V, 5V or adjustable output set via an external resistor divider.
Table 5. Output Voltage Selection
VOUT = 3.3V

FB pin connected to OUT (seeapplication circuit)

VOUT = 5V

FB pin connected to GND

2V ≤ VOUT ≤ 5.2V

FB pin connected to a resistivedivider
R4V OUT = 1.23V ⎛⎝ 1 + --------⎞⎠
R57/13

L6920Figure 12. Demoboard Circuit
Panasonic
ELL6RH100M+VBATT
VBATT
R1N.C.

LBI

C247µF

7GND
8C4100nF

461F.B.

VOUT

VOUT

5C147µF

C3N.C.

R3N.C.

L6920
3GND

Panasonic
EEFCDJ470R

2R2N.C.
VREF+VBATTL1 10µH

Panasonic
EEFCDJ470R
GND

LBO

LBO

SHDN
J1123SHDN
1R4 N.C.

3

R5 N.C.

2J2

not mounted components

D01IN1310

Table 6.
JumperPosition

Function

1-2

Device enabled

2-3

Device disabled

None

Adjustable using R4 and R5 [not mounted]

1-2

3.3V output voltage

2-3

5V output voltage

J1J2

R4, R5 should be selected in the range of 100kΩ - 10MΩ to minimize consumption and error due to current sunk
by FB pin (few nA).
5.2 Output capacitor selection
The output capacitor affects both efficiency and output ripple so its choice has to be considered with particularcare.
The capacitance value should be in the range of about 10µF-100µF.
An additional, smaller, low ESR capacitor can be in parallel for high frequency filtering. A typical value can bearound 1µF.
If very high performances, in terms of efficiency and output voltage ripple, are required, a very low ESR capacitor has to be chosen.
Ceramic capacitors are the lowest ESR but they are very expensive.
Other possibilities are low-ESR tantalum capacitors, available from KEMET, AVX and other sources. POSCAP
capacitors from SANYO and polymeric capacitors from PANASONIC are also good.
Below there is a list of some capacitors suppliers. The cap values and rated voltages are only a suggested possibility
8/13

L6920
Table 7. Capacitors distributors main list
ManufacturerSeries

Cap Value (µF)

Rated Voltage (V)ESR (mΩ)AVX

TPS

15 to 470

6.3

50 to 1500

KEMET

T510/T494/
T495

10 to 470

630 to 1000

PANASONIC

EEFCD

22 to 47

6.3

50 to 700

SANYO POSCAPTPA/B/C

22 to 230

6.340 to 80

SPRAGUE

595D

100 to 390

6.3

160 to 700

5.3 Inductor selection
Usually, inductors ranging between 5µH to 40µH satisfy most of the applications.
Small value inductors have smaller physical size and guarantee a faster response to load transient but in steadystate condition a bigger ripple on output voltage is generated. In fact the output ripple voltage is given by Ipeak
multiplied by ESR. Furthermore, as shown in equation (1), inductor size affects also the maximum current deliverable to the load. Lastly, a low series resistance is suggested if very high efficiency values are needed. Anyway, the saturation current of the choke should be higher than the peak current limit of the device (1A).
Good surface mounting inductors are available from COILCRAFTS, COILTRONICS, MURATA and other souces. In the following table are listed some suggested components.
Table 8. Inductors distributors main list
Manufacturer
CoilcraftSeries

Inductor Value (uH)Saturation Current (A)DO1813HC

22 to 33

1 to 1.2

DO16084.7 to 15

0.9 to 1.5

UP1B

22 to 33

1 to 1.2

TP3

4.7 to 150.97 to 1.6

HM76-2

22 to 33

1 to 1.2

HM76-1

4.7 to 10

1 to 1.5

Murata

LQN6C

10 to 22

1.2 to 1.7

Panasonic

ELL6SH

10 to 22

0.9 to 1.5

ELL6RH

5.1 to10

11 to 1.55

CR43

4.7 to 100.84 to 1.15

Coiltronics

BISumida

5.4 Layout Guidelines
The board layout is very important in order to minimize noise, high frequency resonance problems and electromagnetic interference.
It is essential to keep as small as possible the high switching current circulating paths to reduce radiation andresonance problems. So, the output and input cap should be very close to the device.
The external resistor dividers, if used, should be as close as possible to the pins of the device (FB and LBI) andas far as possible from the high current circulating paths, to avoid pick up noise.
Large traces for high current paths and an extended groundplane, help to reduce the noise and increase theefficiency.
For an example of recommended layout see the following evaluation board.

9/13

L6920Figure 13. Demoboard Components (Top side).

4.5cm

4cm

Figure 14. Demoboard Layout (Top side).

4.5cm

4cm

Figure 15. Demoboard Layout (Bottom side).

4.5cm

4cm

10/13

L6920

6Package Information

Figure 16. TSSOP8 Mechanical Data & Package Dimensions
mminch

DIM.
MIN.

TYP.

A

MAX.

MIN.

TYP.

1.20

A1

0.050

A2

0.800

bMAX.
0.047

0.150

0.002

1.050

0.031

0.190

0.300

0.007

0.012

c

0.090

0.200

0.003

0.008

D (1)

2.900

3.000

3.100

0.114

0.118

0.122

E

6.200

6.400

6.650

0.244

0.252

0.260

E1 (1) 4.300

4.400

4.500

0.169

0.173

0.177

eL
L1kaaa

1.000

0.650
0.450

0.600

OUTLINE AND
MECHANICAL DATA

0.006
0.039

0.041

0.026
0.750

0.018

1.000

0.024

0.027

0.039
0˚ (min.) 8˚ (max.)
0.100

0.004

Note: 1. D and F does not include mold flash or protrusions.
Mold flash or potrusions shall not exceed 0.15mm(.006inch) per side.

TSSOP8(Body 4.4mm)

0079397 (Jedec MO-153-AA)

11/13

L6920

7

Revision History

Table 9. Revision History
Date

Revision

May 2003

1First Issue.

February 2005

2

Modified the max. value of the Isd parameter in the Table 4 pag. 3.

12/13Description of Changes

L6920Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequencesof use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are notauthorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
The ST logo is a registered trademark of STMicroelectronics.
All other names are the property of their respective owners© 2005 STMicroelectronics - All rights reserved
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