ST AN1941 Application note
AN1941
APPLICATION NOTE
LOW VOLTAGE LED DRIVER USING
L6920D, L4971 AND L6902D
1 INTRODUCTION
High brightness LEDs are becoming a prominent source of light and often have better efficiency 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 energy source not only for the reduction of losses, but also for the lumen maintenance of the LED itself. STMicroelectronics has developed the following non-isolated DC-DC constant current LED driver to aid designers in developing a low cost and efficient platform for driving high brightness LEDs.
This application note will cover 3 DC-DC power supplies to drive high intensity LEDs.
1The L6920D boost converter to drive 1 LED for a flash light application
2The L4971 buck converter to drive 1 to 9 LEDs
3The L6902D buck converter to drive 1 to 6 LEDs
Figure 1. Reference Design Boards:
L6920D |
L4971 |
L6902D |
AN1941/0604 |
1/15 |
AN1941 APPLICATION NOTE
2 L6920D LED DRIVER
White LEDs are gaining popularity as sources of illumination because of their high efficiency and reliability. Typical forward voltage drop of a white LED is approximately 3.5V. When these LEDs are powered from a single or two cell batteries, a boost converter is needed to boost the voltage to drive the LEDs.
2.1 L6920D Description
L6920D is a high efficiency step-up converter requiring very few external components to realize the conversion from the battery voltage to the selected output voltage or current. The startup is guaranteed at 1V and the device is operating down to 0.6V. The device has very low quiescent current, only 10µA. Internal synchronous rectifier is implemented with a 120mΩ P- channel MOSFET, replacing the conventional boost diode, to improve the efficiency. This also implies a reduced cost in the application since no external diode required.
Following is the block diagram of L6920D.
Figure 2. Block diagram of L6920D
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OUT |
VOUT |
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ZERO CROSSING |
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- |
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VREF |
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+ |
- + |
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+ |
VBG |
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SHDN |
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FB |
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Y |
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VOUT |
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VOUT |
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A |
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LX |
VIN |
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GND |
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B |
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A |
R1,R2 |
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C |
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OPAMP |
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Y |
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B |
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(CR) |
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C |
+ |
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VBG |
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Q |
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GND |
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Toff min |
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1 sec |
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R |
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CURRENT LIMIT |
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LBO |
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Ton max |
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VBG |
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5 sec |
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LBI |
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D99IN1041 |
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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 bottom MOFSET Q1 is turned on, and the inductor is charged. The switch 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, and 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.
2/15
AN1941 APPLICATION NOTE
2.2 Circuit Description
The circuit shown in figure 3 is a constant current control to provide constant luminosity from the LED. A current sensing resistor is in series with the white LED is used to provide the current feedback. The feedback reference voltage for the controller is 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 sense resistor, R1 to reduce the dissipation and an op-amp to amplify the current sense voltage back up to the required 1.23V level.
Figure 3. Schematic of L6920D LED driver
J2 |
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CON1 |
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U1 |
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1 |
L1 |
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7 |
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8 |
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LX |
OUT |
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C2 |
10uH |
5 |
SHDN |
FB |
1 |
C3 |
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.47uF |
47uF |
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47uF |
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2 |
LBI |
LBO |
3 |
6.3V |
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6.3V |
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4 |
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6 |
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REF |
GND |
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J6 |
C4 |
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L6920 |
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1 |
.1uF |
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U2 TS951ILT |
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CON1 |
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+ |
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OUT |
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OPAMP |
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C5 |
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.01uF |
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R2 |
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R3 |
1 |
2 |
1 |
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100K |
12K |
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1/8W |
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1/8W |
1 |
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1 |
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J7 |
J8 |
CON1 |
CON1 |
J4
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CON1 |
D1 |
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J5 |
LED |
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1 |
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2 |
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R1 |
CON1 |
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.33 Ohm |
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1 |
1/4W |
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2
2
R4
1K
1/8W 1
From the circuit, the control rule is: ILED·R1·K = Vref
where ILED is the current through the LED; R1 is the current sensing resistor, K is the gain of the OP AMP, and Vref is the reference voltage.
Therefore, the LED current will be ILED = |
VREF |
R-----1--------K--- |
In the reference circuit, there are two gains. When J7 and J8 are 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 ILED1 = 280mA or ILED2 = 32 mA.
Following are some typical waveforms at Vin=2.5 V.
3/15
AN1941 APPLICATION NOTE
Figure 4. Upper trace: inductor current; lower track: LED current
IL 500mA/div
ILED 100mA/div
Figure 5. Upper trace: inductor current; lower track: LED current
IL 500mA/div
ILED 100mA/div
from the waveforms, the inductor peak current is limited at 1A. the maximum load current is defined by following relationship:
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Vin |
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Vout – Vin |
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η |
load_lim |
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lim |
off min |
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Vout |
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2 L |
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where η is the efficiency, Ilim =1A, and Toffmin =1µs.
When the load is heavier than Iload_lim, the regulation will be lost, and the inductor current will go to continuous mode. Fig. 6 and Fig. 7 show that the circuit loses the regulation, but the cir-
cuit is running at its maximum duty cycle.
Figure 6. Vin = 1V; upper trace: inductor current; lower trace: LED current
IL 500mA/div
ILED 100mA/div
4/15
AN1941 APPLICATION NOTE
Figure 7. Vin = 0.6V; upper trace: inductor current; lower trace: LED current
IL 500mA/div
ILED 100mA/div
Fig. 8 shows the efficiency of the driver at different load and input voltages.
Figure 8. Efficiency curve
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Efficiency |
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1 |
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(%) |
0.9 |
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Efficiency |
0.8 |
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0.7 |
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0.6 |
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0.5 |
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1.9 |
2.1 |
2.3 |
2.5 |
2.7 |
3 |
Input Voltage (V)
275mA Output
30mA Output
Table 1. Bill of Material:
Ref |
Value |
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C2,C1 |
47uF 6.3V Electro sm |
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C3 |
.47uF 0805 |
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C4 |
.1uF 0805 |
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C5 |
.01uF 0805 |
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L1 |
10uH sm inductor |
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R1 |
.33 Ohm 1% 1/4W 0805 |
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R2 |
100K 5% 0805 |
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R3 |
12K 5% 0805 |
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R4 |
1K 5% 0805 |
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U1 |
L6920D Tssop8 |
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U2 |
TS951ILT sot23 |
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5/15
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