AL5801

Document number: DS35555 Rev. 3 - 2

9 of 11

www.diodes.com

July 2012

© Diodes Incorporated

AL5801

PWM Dimming

(a)

(b)

Figure 16 Application Circuits for LED Driver with PWM Dimming Functionality (a) MOSFET driving and (b) Transistor driving

PWM dimming can be achieved by driving the BIAS pin (1). An external open-collector NPN transistor or open-drain N-channel MOSFET can be

used to drive the BIAS pin as shown in Figure 16. Dimming is achieved by turning the LEDs ON and OFF for a portion of a single cycle. The

PWM signal can be provided by a micro-controller or by analog circuitry.

Figure 17 shows the LED current against the PWM signal duty ratio when the AL5801 is used to drive three series connected LEDs from a 12V

supply. The PWM dimming frequency is set to 200Hz. The PWM signal is supplied to the open-Drain small signal MOSFET’s gate as shown in

Figure 16a. The BIAS pin signal is an inversion of the PWM drive to the MOSFET’s gate. Therefore, a PWM signal duty cycle of 0% provides the

maximum LED current. Sufficiently large PCB copper area is used for heat sinking of the AL5801 in order to minimize the device self-heating at

Figure 17 LED Current against PWM Dimming Signal Duty Ratio at 200Hz PWM Frequency