PIC voltage doubler

[Mosaic]

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?

thx
Ancel

 

[JonSea]

The EEVBlog had a couple videos talking about these. Have a look and you'll probably learn everything you need to know.

 

[Overclocked]

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).

 

[Mosaic]

Sims of the doubler creates massive ripple under loads. The inductor approach yields reasonable results..
Here is a sim using a 2n2222 to charge the inductor and a low ESR 220uf smoothing cap coupled with a zener voltage limiter.
https://www.jameco.com/webapp/wcs/s...-1&catalogId=10001&pa=606820&productId=606820

The sim. shows around 6Volts at the 200mA with good ripple due to the 200Khz PWM.

I selected an inductor for its average current, but I'd like some advice on its suitability as it's called an RF choke.
https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_642492_-1

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.

 

[Overclocked]

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.

 

[Mosaic]

I found this to be an excellent resource:
https://www.analog.com/static/imported-files/tech_articles/MS-2208.pdf
And this calc:
https://www.daycounter.com/Calculators/Switching-Converter-Calculator.phtml

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.

 

[Pommie]

You may find this thread interesting.

Mike.