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

I	I

Imax Iaverage

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 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).

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.

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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)

							Vcc
			VOLTAGES
			MONITOR
	INHIBIT			3.8V			VREF	Vref
					SUPPLY
				1.235V			GOOD
CS+	Current_E/A
	+		THERMAL
	-		SHUTDOWN		TRIMMING
CS-
COMP				PEAK TO PEAK
		Voltage_E/A		CURRENT LIMITING
VFB	1.235V	+	-
		E/A			D	Q
		-	+PWM		CK		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] 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

U1

L6902D

J2

R1

C2

1

+

2

8

VCC

OUT

1

D1

9k1

10uF

2

1

STPS34OU

35V

8 - 24V

CS+

2

Output

4

3

C1

COMP

VREF

CS-

1k

R4

+

22nF

C4

GND

FB

5

25V

10uF

C3

CERAMIC

220pF

7

6

R2

R3

R6

R5

Rsense

5k1

510

27k

8k2

0.33

1			1	1

J3	J4	J6 GND
3.3V Vref	Dimming Input

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