ST AN1941 Application note

AN1941

APPLICATION NOTE

LOW VOLTAGE LED DRIVER USI N G

L6920D, L4971 AND L6902D

1 INTRODUCTION

High brightness LE Ds are becoming a pr ominent sou rce of li ght and often have be tter eff iciency and reliability when compared to that of conventional light sources. While LEDs can operate from an energy source as simple as a battery and resistor, most applications require an efficient e nergy sour ce not onl y for t he red uction of l osses, bu t als o for the l umen mainten ance of the LED itself. STMicroelectronics has developed the following non-isolated DC-DC constant current LED driver to aid designers in developin g a low cost and efficient pl atform for dri ving high brightnes s LEDs.

This application note will cover 3 DC-DC power supplies to drive high intensity LEDs.

1 The L6920D boost converter to drive 1 LED for a flash lig ht application 2 The L4971 buck converter to dr ive 1 to 9 LEDs 3 The L6902D buck converter to dr ive 1 to 6 LEDs

Figure 1. Reference Design Boards:

AN1941/0604

L6920D L4971 L6902D

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AN1941 APPLICATION NOTE

2 L6920D LED DRIVER

White LEDs are gaining popularity as sources of illumination because of their high efficiency and reliabili ty. Typical forwar d volt age dr op of a wh ite LED i s appr oximat el y 3.5V. When these LEDs are power ed fr om a singl e or two cell batt erie s, a boos t convert er i s needed t o boost the voltage to drive the LEDs.

2.1 L6920D Descripti on

L6920D is a hig h efficiency step-up converter requiring very few ext e rnal components to realize the conversio n from the batt ery voltag e to the selected output volt age or curren t. The startup is guaranteed at 1V and the device i s operating down to 0.6 V. The device has very low qui escent current, only 10µA. Internal synchronous rectifier is implemented with a 120mΩ Pchannel MOSFET, replaci ng the convent ional boos t diode, to impro ve the effi ciency. This al so implies a reduced cost in the application since no external diode required.

Following is the block diagr am of L6920D.

Figure 2. Block diagram of L6920D

OUT

REF

SHDN

LBO

OUT

ZERO CROSSING

VBG

FB Y

OUT

GND

VBG

1,R2

Toff min

1µsec

VBG

OPAMP

(CR)

LBI

RQS

A B

Ton max

5µsec

- +

CURRENT LIMIT

VOUT

GND

D99IN1041

In L6920D, the control is based on the comparator that continuously checks the status of the feedback signal. If the feedback voltage is lower than reference value, the control function of the L6920D 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 botto m MOFSET Q1 is t urn ed on, an d the i nduc tor is c harg ed. The s wit ch

is turned off if the current reaches 1A or after a maximum on-time set to 5s.

– Toff phase: the bottom MOSFET Q1 is turned off, a nd top MOSFET Q2 is turned on. The

energy stored in the inductor is transferred to the load for at least a minimum off time of 1s. After this, the synchronous switch is turned off as soon as the feedback signal goes lower than reference or the current flowing in the inductor goes down to zero.

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AN1941 APPLICATION NOTE

2.2 Circuit Description

The circuit shown in fi gure 3 is a constant current control to provide constant luminosity from the LED. A current sensing res istor is in series with the white LED is used to provi de the current feedback. Th e feedback r eference voltage for the controller i s 1.23V. If this voltage level is directly feedback from the current sensing resistor, the loss in the resistor will be too high. The circuit uses a low value se nse re sist or, R1 to reduce t he dissi pati on and an op-amp to ampl ify the current sense vol tage back up to the required 1.23V level.

Figure 3. Schematic of L6920D LED driver

CON1

10 uH

47uF

6.3V

SHDN

LBI

4 6

REF GND

L6 920

C4 .1u F

OUT

LBO

.33 O hm

1 2

1/4W

CON1

LED

.47uF

C1 47uF

6.3V

OPAMP

U2 TS95 1ILT

OUT

.01uF

From the circuit, the control rule is: I where I

is the current through the LED; R1 is the current sensing resistor, K is the gain of

LED

·R1·K = Vref

LED

J7 CON1

1 2

100K

1/8W

J8 CON1

1 2

12K

1/8W

1 2

the OP AMP, and Vref is the reference voltage.

REF

Therefore, the LED current will be

LED

----------------=

R1 K⋅

In the reference ci rcuit, there are two gains. When J7 and J8 ar e shorted, K1= 1+R3/R4. When J7 and J8 are open, K2=1+(R3+R2) /R4.

In the circuit, R1 = 0.33Ω; R2 = 100 kΩ; R3 = 12 kΩ; R4 = 1 kΩ. the current level of the LED can be I

= 280mA or I

LED1

LED2

= 32 mA.

Following are some typical waveforms at Vin=2.5 V.

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AN1941 APPLICATION NOTE

Figure 4. Upper trace: inductor current; lower track: LED current

500mA/div

100mA/div

LED

Figure 5. Upper trace: inductor current; lower track: LED current

500mA/div

100mA/div

LED

from the waveforms, the inductor peak current is limited at 1A. the maximum load current is defined by following relationship:

load_lim

where η is the efficiency, I

=1A, and T

lim

When the load is heavier than I

Vin

-------------

Vout

load_lim



lim

offmin

off min



=1µs.

, the regulation will be lost, an d the inductor current will

Vout Vin–

-----------------------------

⋅

2L⋅

η⋅–⋅=

go to continuous mode. Fig. 6 and Fig. 7 show that the circui t loses th e regulatio n, but the circuit is running at its maximum duty cycle.

Figure 6. Vin = 1V; upper trace: inductor current; lower trace: LED current

500mA/div

100mA/div

LED

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AN1941 APPLICATION NOTE

Figure 7. Vin = 0.6V; upper trace: inductor current; lower trace: LED current

500mA/div

100mA/div

LED

Fig. 8 shows the efficiency of the driver at dif ferent load and input voltages.

Figure 8. Efficiency curve

Efficiency

0.9

0.8

0.7

0.6

Efficiency (%)

0.5

1.92.12.32.52.7 3

Input Voltage (V)

275mA Output

30mA Output

Table 1. Bill of Material :

C2,C1 47uF 6.3V Electro sm C3 .47uF 0805 C4 .1uF 0805 C5 .01uF 0805 L1 10uH sm inductor R1 .33 Ohm 1% 1/4W 0805 R2 100K 5% 0805 R3 12K 5% 0805 R4 1K 5% 0805 U1 L6920D Tssop8 U2 TS951ILT sot23

Ref Value

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