Diodes AP8801 - Manual (Page 9)

AP8801
500mA LED STEP-DOWN CONVERTER
AP8801
Document number: DS31765 Rev. 7 - 2
9 of 14
www.diodes.com
June 2012
© Diodes Incorporated
Application Information (cont.)
PWM Dimming of LED Current
When a low frequency PWM signal with voltages between 2.5V and a low level of zero is applied to the CTRL pin the output current will be
switched on and off at the PWM frequency. The resultant LED current I
LEDavg
will be proportional to the PWM duty cycle. See figure 18.
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. The ultimate resolution will be determined by the number of
switching cycles required to get back to nominal LED current once the PWM voltage is high relative to PWM frequency. Lower switching
frequencies and higher PWM frequencies will result in lower PWM dimming dynamic ranges.
Figure. 18 Low Frequency PWM Operating Waveforms
There are different ways of accomplishing PWM dimming of the AP8801 LED current:
Directly Driving CTRL Input
A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the CTRL pin to adjust the output current to a value above
or below the nominal average value set by resistor R
SET
. When driving the CTRL with a voltage waveform care should be taken not to
exceed a drive voltage of 2.5V (where extra brightness is required) or 1.25V if a maximum of 100% brightness is required.
A way of avoiding over-driving the CTRL pin is use an open collector/drain driver to drive the CTRL pin.
Driving the CTRL Input via Open Collector Transistor
The recommended method of driving the CTRL pin and controlling the amplitude of the PWM waveform is to use a small NPN switching
transistor. This uses the internal pull-up resistor between the CTRL pin and the internal voltage reference to pull-up CTRL pin when the
external transistor is turned off.
Driving the CTRL Input from a Microcontroller
If the CTRL pin is driven by a MOSFET (either discrete or open-drain output of a micro-controller) then Schottky diode maybe be required
due to high Gate / Drain capacitance, which could inject a negative spike into CTRL input of the AP8801 and cause erratic operation but the
addition of a Schottky clamp diode (eg. Diodes Inc. SD103CWS) to ground and inclusion of a series resistor (3.3k) will prevent this.