Ap8803, Application information – Diodes AP8803 User Manual
Page 7
AP8803
Document number: DS31970 Rev. 5 - 2
7 of 10
August 2012
© Diodes Incorporated
AP8803
Application Information
(cont.)
PWM Dimming
A Pulse Width Modulated (PWM) signal with a max resolution of 8-bit, can be applied to the CTRL pin to change the output current to a value
above or below the nominal average value set by resistor R
SET
. To achieve this resolution the PWM frequency has to be lower than 500Hz.
Figure 5 shows the typical PWM response of the AP8800A. An internal filter produces a rump
Figure 5 Typical PWM Dimming Waveform
The recommended method of driving the CTRL pin and controlling the amplitude of the PWM waveform is to use a small NPN switching
transistor as shown below:
Figure 6 Open Collector PWM Dimming Circuit
This scheme uses the internal 200k
Ω resistor between the CTRL pin and the internal voltage reference as a pull-up resistor for the external
transistor eg MMBT3904.
Soft-Start
An external capacitor from the CTRL pin to ground will provide soft-start delay, by increasing the time taken for the voltage on this pin to rise to
the turn-on threshold and by slowing down the rate of rise of the control voltage at the input of the comparator.
The soft-start time is 0.5ms/nF.