AN2129 APPLICATION NOTE

2/9

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)

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 10k

Ω 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).

VREF

GOOD

3.8V

COMP

VFB

DRIVER

INHIBIT

+

-

1.235V

E/A

1.235V

THERMAL

SHUTDOWN

-

+

PWM

OSCILLATOR

OUT

GND

SUPPLY

TRIMMING

FREQUENCY

SHIFTER

VOLTAGES

MONITOR

D

Q

CK

CURRENT LIMITING

PEAK TO PEAK

+

-

Current_E/A

Voltage_E/A

CS-

CS+