AP8800EV1

Issue 3 – August 2009

www.diodes.com

© Diodes Incorporated 2009

6

Circuit features

N.B. Remove power whilst changing components!

Soft-start

1.

The AP8800 has a in-built soft start function. A capacitor, C2 may be fitted to the evaluation

board to increase the soft start time by slowing the rise time of the adjust pin at start-up at the

rate of 0.5ms/nF. The board is supplied with a zero-ohm resistor in position R2. Please see

the data sheet for further details.

PWM

1.

Remove the soft start capacitor C2

2.

Refer to the datasheet for instructions on how to perform PWM

Switching off the output current

3.

Shorting the CTRL pin to GND will cause the LED current to go to zero. Releasing this pin will

switch on the system (creating a soft-start power up sequence if the C2 capacitor is used).

Changing the LED current

1.

Remove R1

2.

Calculate and fit a new sense resistor, R1, the value of which is based on the required LED

current without dimming. R1 can be calculated using following equation :

R1 = the sense resistors value in ohms.

0.1V is the nominal sense voltage with CTRL open circuit or set to

1.25V.

PERFORMANCE

The system efficiency depends on the sense resistor, supply voltage, switching inductor, and the

number of LEDs.

With a 28V supply and 7 LEDs, the switching frequency is typically 320 kHz and efficiency levels

>90% are achievable.

REFERENCE