ST AN2129 Application note
AN2129
APPLICATION NOTE
DIMMING OF SUPER HIGH BRIGHTNESS LEDS
WITH L6902D
1 Introduction
Thanks to the high efficiency and reliability, super high brightness LEDs are becoming more
and more important when compared to conventional light sources. Although LEDs can be supplied directly from a simple voltage source (like battery with resistor), for most applications it
is better to use a switching current source to get not even higher efficiency but also to get a
better light output. This paper will focus on a L6902D based DC/DC converter with dimming
interface. For more details about other converters and applications for LEDs available from
STMicroelectronics please refer to other application notes ([1] and [2]).
2 Dimming Concepts
There are two basic principles how the light output of the LED can be controlled. Since the light
brightness is proportional to the current, both methods are dealing with current regulation. The
first and the easiest way is to control the LED current itself, with the principal sketch in Figure
1, where current is changed proportionally with the dimming signal. Disadvantage of this analog control is that there can be a significant change of color (wavelength difference could be
several nanometers) in deep dimming (less that 10%). This potential disadvantage is compensated by a very simple control circuit (usually a simple potentiometer is enough).
Figure 1. Analog current control Figure 2. Average current control by PWM
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time
The second method is based on an average current control (digital control) as can be seen in
Figure 2. The current is switched between zero and the nominal current with a frequency higher than 100Hz (to avoid flickering). The change of duty cycle and hence the average current
change will be seen as a brightness change, because human eye reaction is slow enough to
"integrate" the light output and it will not be noticed as a blinking.
This method avoids the color change problem, but on the other hand it needs more sophisticated control circuits (usually a microcontroller or another simple PWM generator).
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time
3 L6902D DC/DC Converter
The L6902D is a complete and simple step down switching regulator with adjustable current
and voltage feedback. Thanks to its current control loop with external sense resistor it is able
to work in a constant current mode, providing up to 1A output current with an accuracy of 5%.
Among other features there can be also found general purpose 3.3Volts precise (2%) reference voltage or 2.5A (typical value) internal current limit for short circuit protection.
AN2129/0705
Rev. 2
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AN2129 APPLICATION NOTE
In Figure 3 is the internal structure of the L6902D converter, the datasheet [3] should be referred for more details.
Figure 3. L6902D Block diagram (see [3] for details)
V
cc
VOLTAGES
MONITOR
V
ref
CS+
CS-
INHIBIT
Current_E/A
+
-
THERMAL
SHUTDOWN
3.8V
1.235V
SUPPLY
TRIMMING
VREF
GOOD
COMP
VFB
1.235V
Voltage_E/A
+
E/A
-
OSCILLATOR
-
PWM
+
PEAK TO PEAK
CURRENT LIMITING
D
CK
GND
Q
DRIVER
OUT
FREQUENCY
SHIFTER
4 Application Board
An application board using the dimming principles described above has been designed and its
schematic is in Figure 5. There is only a single dimming input connector on the board; usable
for both dimming methods (either analog or PWM control can be used, as preferred). There
were made some changes compared to the application circuit presented in datasheet [3] allowing this dimming. First of all, the sense resistor has been moved from higher voltage path
(coil output) to the lower one (output ground). Then three resistors were added (R4, R5 and
R6) for modifying the current sense feedback.
A signal between 0 and 3.3V should be used for analog (peak current) dimming. When the
dimming pin is grounded (0V) the maximum output current is provided (350mA) and vice versa
when 3.3V is applied to the pin, the current provided is zero and so the LED is off. There are
two more pins on the board: 3.3V reference voltage pin and ground pin (a jumper can be used
to connect the dimming pin to the ground pin for the maximum output). For the easiest way of
dimming just connect the 10kΩ potentiometer between 3.3V and ground pins. The potentiometer slider should be connected to the dimming pin (as it can be seen in Figure 4).
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AN2129 APPLICATION NOTE
Figure 4. Connecting the potentiometer for analog dimming
The second dimming method implemented on this board is a PWM control of average LED current. This control needs a digital PWM signal (amplitude can be either 3.3V or 5V) between
dimming pin and ground pin. Then varying the duty cycle will change the LED brightness
(100% means LED off and 0% means LED fully on).
With the closer look on the application (Figure 5) it is noticeable that cathode of the LED must
not be connected to the ground of the circuit, because there is a sense resistor between cathode and the ground. If by any accident, LED cathode is grounded, the current feedback loop
will be inactivated and the L6902D will set the maximum output voltage (as set by the voltage
divider R1 and R3) regardless the current which can eventually destroy the LED. Also care
must be taken on input voltage polarity together with output LED polarity. If the input polarity
is twisted, the whole IC could be damaged. While with the output polarity reversed, the board
itself cannot be damaged, but the LED will see the maximum voltage (as limited by the voltage
divider R1 and R3) in reverse direction.
Figure 5. Board schematic (order code STLEDDCDIM-EVAL1)
L1
100uH
J1
8 - 24V
2
1
C1
+
10uF
25V
CERAMIC
R2
5k1
C3
22nF
C4
220pF
L6902D
U1
8
VCC
4
COMP
GND
7
OUT
CS+
CS-
VREF
6
1
J3
3.3V Vref
1
2
3
5
FB
D1
STPS34OU
Rsense
0.33
J2
1
2
Output
1
J6
GND
R1
9k1
R3
510
R6
27k
1
J4
Dimming Input
C2
+
10uF
35V
1k
R4
R5
8k2
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