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 efficien-
cy and reliability when compared to that of conventional light sources. While LEDs can oper-
ate 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 con-
stant current LED driver to aid designers in developing a low cost and efficient platform for driv-
ing high brightness 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 light application
2 The L4971 buck converter to drive 1 to 9 LEDs
3 The 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 real-
ize the conversion from the battery voltage to the selected output voltage or current. The start-
up is guaranteed at 1V and the device is operating down to 0.6V. The device has very low qui-
escent 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
OUT
ZERO CROSSING
V
REF
SHDN
FB
V
OUT
GND
R
1
,R
2
Y
-
+
VBG
Q
S
R
-
+
CURRENT LIMIT
LBO
-
+
VBG
Ton max
5µsec
FB
GND
Y
A
B
C
OPAMP
(CR)
VOUT
LX
V
IN
-
+
VBG
-
+
-+
V
OUT
A
B
C
Toff min
1µsec
LBI
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 ac-
complished 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 di-
rectly 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
CON1
L1
7
+
U1
LX
SHDN
LBI
REF
OUT
FB
LBO
GND
8
1
3
6
2
1
1
J4
C3
.47uF
+
1
C2
47uF
6.3V
10uH
5
2
4
C1
47uF
6.3V
CON1
J5
R1
D1
LED
J6
1
C4
.1uF
L6920
U2 TS951ILT
OUT
1
+
.33 Ohm
1/4W
CON1
CON1
OPAMP
C5
.01uF
R2
1
-
R3
2
1
100K
1/8W
12K
1/8W
2
2
R4
1K
1/8W
1
1
J7
CON1
J8
CON1
From the circuit, the control rule is: I
LED
·R1·K = Vref
where I
LED
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.
REF
Therefore, the LED current will be
I
LED
= ----------------
V
R1
⋅
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 I
LED1
= 280mA or I
LED2
= 32 mA.
Following are some typical waveforms at Vin=2.5 V.
1
3/15
AN1941 APPLICATION NOTE
Figure 4. Upper trace: inductor current; lower track: LED current
I
L
500mA/div
I
LED
100mA/div
Figure 5. Upper trace: inductor current; lower track: LED current
I
L
500mA/div
I
LED
100mA/div
from the waveforms, the inductor peak current is limited at 1A. the maximum load current is
defined by following relationship:
Vin
-
Vout
–
Vin
-
I
load_lim
= ------------
⋅
I
lim
–
T
off min
⋅
----------------------------
⋅ η
Vout
2
⋅
L
where
η
is the efficiency, I
lim
=1A, and T
offmin
=1µs.
When the load is heavier than I
load_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
I
L
500mA/div
I
LED
100mA/div
4/15
AN1941 APPLICATION NOTE
Figure 7. Vin = 0.6V; upper trace: inductor current; lower trace: LED current
I
L
500mA/div
I
LED
100mA/div
Fig. 8 shows the efficiency of the driver at different load and input voltages.
Figure 8. Efficiency curve
Efficiency
1
Efficiency (%)
0.9
0.8
0.7
0.6
0.5
1.9
2.1
2.3
2.5
2.7
3
Input Voltage (V)
275mA Output
30mA Output
Table 1. Bill of Material:
Ref
C2,C1
C3
C4
C5
L1
R1
R2
R3
R4
U1
U2
.47uF 0805
.1uF 0805
.01uF 0805
10uH sm inductor
.33 Ohm 1% 1/4W 0805
100K 5% 0805
12K 5% 0805
1K 5% 0805
L6920D Tssop8
TS951ILT sot23
Value
47uF 6.3V Electro sm
5/15
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