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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)

VCC

8

COMP

4

OUT

1

CS+

2

CS-

3

FB

5

U1

L6902D

R5

8k2

R6

27k

J4

Dimming Input

+

C1

10uF

25V

CERAMIC

C4

220pF

C3

22nF

R2

5k1

STPS34OU

D1

L1

100uH

+

C2

10uF

35V

R1

9k1

R3

510

J6 GND

J3

3.3V Vref

Rsense

0.33

1

2

J1

8 - 24V

1

2

J2

Output

R4 1k