DT0013
Design tip
L6920 as LED driver for flashlight
By Nichola Siciliano
Jianwen Shao
L6980DB
TS951ILT
STR2N2VH5
Main components
Synchronous rectifier step up converter
Input/output rail-to-rail low-power operational amplifiers
N-channel 20 V, 0.025 Ω typ., 5 A
Purpose and benefits
This design tip describes how to implement a solution for high current LED drivers that are suitable for those applications where the available power supply voltage is lower than the
LED forward voltage. This kind of solution can be used, for instance, in a flashlight in which the regular bulb is to be replaced with a high current LED device (350mA typical). In this case, a boost topology implementation is mandatory.
In battery powered applications, the input voltage varies with the battery discharge profile.
The following are the suitable battery types for these kinds of applications:
- 2 Alkaline cells AA or AAA
- 1 Li-Ion cell, with the voltage range comprised between 4.2V and 2.7V or less
- 2 Ni-MH cells, with the voltage range between 1.5V to 0.9V
We will focus our application examples on the L6920DB step-up switching regulator.
December 2013 DT0013 Rev 1 1/5
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Description
The schematic in Figure 1 shows a basic circuit in which the L6920DB is configured as a constant current source to drive the high brightness LED. An output current of 350mA is set by the resistor, Rsense, according to the formula:
Rsense = Vref/ I
LED where Vref is 1.23V
Figure 1. LED driver: basic configuration
3V
BATTERY
+
L1 10uH
C1
47uF
C2
0.1uF
5
7
2
4
U1
SHDN OUT
LX
LBI
REF
FB
LBO
GND
8
1
3
6
L6920D
+
C3
1uF
LED
Rsense
0.28 R
In this basic configuration, the power dissipated in Rsense negatively impacts the system efficiency. Since in battery powered equipment the efficiency is an important parameter, it would be desirable to improve the efficiency. The total power out is the sum of the power in the LED plus the power in Rsense.
Power out = I
LED
*Vf + I
LED
2
*R sense sense
So, lowering the value of Rsense will reduce the wasted power.
The example in Figure 2 shows a circuit to improve the efficiency.
Using an op-amp in a non-inverting configuration, we can decrease the drop across the resistor from 1.25 (reference value), to a significantly lower value. In this way, the LED brightness remains the same but system efficiency is improved. The resistor value depends on:
- The LED forward current (I
LED
)
- The gain stage amplification (A)
A=1+R
3
/R
4
Therefore, Rsense can be calculated from the equation: :
December 2013
R sense
= 1.23V /A*I
LED
DT0013 Rev 1 2/5
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In the schematic of Figure 2, we set the gain at 11.7 so the drop across Rsense is reduced from 1.23V to 105mV, thus the power dissipated in the resistor goes from:
P
Rsense
=I
LED
* 1.23 , (Basic configuration-Fig. 1) to
P
Rsense
=I
LED
* 0.105 , (Higher efficiency configuration-Fig. 2)
The LED considered is 1W (such as the Lumiled Luxeon Emitter, Osram LW-W5SG, or
Seoul Semiconductor W11190), with a forward current in the range of 350mA, therefore
Rsense value is:
R sense
= 0.105V / 0.35A = 0.3Ω
Figure 2. LED driver: Higher efficiency configuration
L1 10uH
3V
+
C1
47uF
R1 1K
Q1
STR2N2VH5
R2 3K
U1
5
7
2
4
SHDN
LX
LBI
OUT
FB
LBO
REF GND
C2
0.1uF
L6920D
8
1
3
6
C3
1uF
+
1
U2
R3
2
5
+
-
3
4
TS951
10.7k
C4 1nF micro or push-button
Shout-Down Stage
0V = System Off
3V = system ON
Gain Stage A= 11.7
R4
1k
This schematic can be easily adjusted to meet other application requirements such as lower or higher LED forward current, higher input voltage range etc...
LED
Rsense
0.3
December 2013 DT0013 Rev 1 3/5
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Support material
Related design support material
Evaluation board – STEVAL-ISA077V21
High efficiency synchronous rectifier step-up converter based on L6920DB
Documentation
Datasheet: L6920DB , Synchronous rectifier step up converter
Datasheet: TS951ILT , Input/output rail-to-rail low-power operational amplifiers
Datasheet: STR2N2VH5 , N-channel 20 V, 0.025 Ω typ., 5 A
AN1941 - Low voltage LED drivers using L6920D, L4971 AND L6902D
AN1891 - Application ideas: driving LEDs using L497x, L597x, L692x DC-DC converters families
Revision history
Date
17-Dec-2013 1
Version
Initial release
Changes
December 2013 DT0013 Rev 1 4/5
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