AN26 Datasheet, PDF (8/8 Pages) Copal Electronics – W-6137EVAL1 EVALUATION BOARD FOR W-6137

AN26
Figure 7. LED Current Waveform with PWM
b) Dimming using a DC voltage applied to the FB pin
A variable external DC voltage can be applied on FB pin to adjust the LED current. As the DC voltage is increased, the
voltage drop on resistor R3 is increased and the voltage drop on RSET = R1+R2 is decreased, thus the LED current
decreases. The external DC voltage is applied to FB pin through a series resistor, R5, which sets the maximum DC
voltage.
Also, a filtered PWM signal can be considered as a variable DC voltage.
- Connect the SHDN pin to VIN: jumper shunt between Pin #1 and Pin #2 of J1 connector.
- Set the ILED current (i.e. ILED_MAX = 20 mA).
- Connect the GEN/DC to FB pin through R5 resistor using one jumper between Pin #3 and Pin #4 and another jumper
shunt between Pin #5 and Pin #6 of J1 connector.
- Apply the variable DC voltage between GEN/DC (T3) and GND (T4).
- Increase the DC voltage value (from 0V to VMAX = 3.2V).
- Observe the ILED current decreases from the ILED MAX (20mA – previous set) to ILED MIN (0mA for VMAX =
3.2V with the existing resistors values, R3 = 1Kohm, R5 = 10Kohm).
© 2005 by NIDEC COPAL ELECTRONICS CORP.
Characteristics subject to change without notice
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Document No. 6043, Rev. A