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 sup-
plied 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 ana-
log 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 compen-
sated by a very simple control circuit (usually a simple potentiometer is enough).
Figure 1. Analog current control
I
Figure 2. Average current control by PWM
I
I
max
I
average
time
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 high-
er 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 sophisti-
cated control circuits (usually a microcontroller or another simple PWM generator).
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%) refer-
ence voltage or 2.5A (typical value) internal current limit for short circuit protection.
AN2129/0705
Rev. 2
1/9
AN2129 APPLICATION NOTE
In Figure 3 is the internal structure of the L6902D converter, the datasheet [3] should be re-
ferred for more details.
Figure 3. L6902D Block diagram (see [3] for details)
V
cc
VOLTAGES
MONITOR
INHIBIT
Current_E/A
+
-
THERMAL
SHUTDOWN
3.8V
1.235V
CS+
SUPPLY
VREF
GOOD
V
ref
CS-
COMP
TRIMMING
Voltage_E/A
VFB
1.235V
+
E/A
-
-
+
PWM
PEAK TO PEAK
CURRENT LIMITING
D
CK
Q
DRIVER
OSCILLATOR
OUT
FREQUENCY
SHIFTER
GND
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] al-
lowing 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 potentiom-
eter 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 cur-
rent. 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 cath-
ode 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
2
1
8 - 24V
+
C1
10uF
25V
CERAMIC
C3
22nF
R2
5k1
8
U1
VCC
L6902D
OUT
CS+
CS-
FB
1
2
3
5
R4 1k
D1
STPS34OU
R1
9k1
+
C2
10uF
35V
1
2
J2
Output
4
C4
220pF
7
6
VREF
GND
COMP
R3
510
R6
27k
R5
8k2
Rsense
0.33
1
1
J3
3.3V Vref
J4
Dimming Input
J6
1
GND
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AN2129 APPLICATION NOTE
Figure 6. PCB layout
Table 1. Bill of materials
Type
Ceramic Capacitor
Tantal Capacitor
Capacitor SMD 0805
Capacitor SMD 0805
Schotky Diode
Connector
Connector
Connector
Connector
Connector
Coil
Resistor SMD 2010
Resistor SMD 0805
Resistor SMD 0805
Resistor SMD 0805
Resistor SMD 0805
Resistor SMD 0805
Resistor SMD 0805
Converter
C1
C2
C3
C4
D1
J1
J2
J3
J4
J6
L1
Rsense
R1
R2
R3
R4
R5
R6
U1
Reference
10uF
10µF; 35V
22nF
220pF
STPS340U
8 -24V
Output
3.3V Vref
Dimming Input
GND
100µH; 1.2A; 0.33Ω
0.33
9k1
5k1
510
1k
8k2
27k
L6902D
Part
N/A
N/A
N/A
N/A
STMicroelectronics
N/A
N/A
N/A
N/A
N/A
Würth Elektronik 744 562 0
N/A
N/A
N/A
N/A
N/A
N/A
N/A
STMicroelectronics
Supplier Order Code
The calculation of the resistor current feedback network can look relatively complicated, but
with few simplifications it becomes easy to take in. First assumption is that all the current flows
only through the R
sense
(i.e. neglecting voltage drop on the resistors R4,R5 and R6); the value
of R
sense
is defined by the output current and the threshold voltage on CS+ pin (100mV). Un-
fortunately this calculation will give uncommon values (e.g. for 350mA it gives 0.2857Ω) thus
the nearest higher standard (e.g. E24 series) value for Rsense should be selected (e.g. 0.33Ω)
and then the difference between ideal and standard value is compensated by R4, R5 and R6
to receive precise output current.
The application is shifting between two limit states with dimming; maximum current (zero dim-
ming voltage) and zero current (full dimming voltage). In Figure 7, the dimming network with
grounded dimming input (Equation 1 describes the circuit) is shown, it means when the current
flowing through the LED is on its maximum (i.e. 350mA on this board).
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AN2129 APPLICATION NOTE
Figure 7. Dimming network with zero dimming voltage (maximum current)
V
dimm
= 0V
R5
⋅
R6
⋅
I
LED
⋅
R
sense
-
100mV
= --------------------------------------------------------------------------
R4
⋅
R5
+
R4
⋅
R6
+
R5
⋅
R6
Eq 1
The second limit state is depicted in Figure 8. In this case the current through the Rsense is
zero (LED is off) and thus on point A there is a zero voltage (i.e. ground). The Equation 2
shows the calculation for this state.
Figure 8. Dimming network with maximum dimming voltage (zero output current)
V
dimm
= 3.3V
R4
⋅
R5
-
100mV
=
V
dimMAX
--------------------------------------------------------------------------
R4
⋅
R5
+
R4
⋅
R6
+
R5
⋅
R6
Eq 2
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