Hi all:
I have a 3.6V Li ion 1400Mah battery supply but I need to run some multiplexed 3.5Vf LEDs so I need to use a pic output pin as a voltage doubler to provide some headroom for batt discharge.
Prob is I need about a 200ma supply from it.
Is there a method using bipolar transistors (2n2222/2907) to achieve this level of current?
Easiest Way to do it would be to use a boost converter for the LEDs and have a transistor on the output of the PIC for selecting the LEDs. You can use a voltage doubler, but I always thought they were only good for a few mA (double the voltage, half the current).
Edit: the LTspice file shows the eff % to be around 80% with a 0.6% ripple which isn't bad for a bipolar switch. No attempt at V reg is made....although there is a limiting Zener, which doesn't do much unless the load is removed. It draws about 9mA peak to limit the boost V then.
For experimentation its fine, but once you start restricting yourself (ie Battery power, efficiency, etc) you'll want to use something with a lower DC resistance and a specialized IC for LED's. I have used those inductors with a "dummy" 555 boost converter.
Look at NCP3065. Simple LED boost converters. The IC's are cheap as well.
Insofar as using a custom IC for the job. I prefer lever to leverage the PIC PWM as this is only about a 200mA requirement. I can use the space (it's a bit tight) saved for larger inductor & caps for better results.