dknguyen

Interesting little things...never had to use one before but I came by a bootstrap capacitor high-side FET driver that required one. It is used as a 15V floating charge pump to refresh the boostrap capacitor in order to allow for continuous duty cycles.

It appears that dv/dt can transients mess around with the transformer interwinding capacitance in isolated floating supplies and I couldn't seem to find a 15V floating charge pump IC so I had to go over how a 555 works.

I never knew their comparators were set to 1/3Vcc and 2/3Vcc. Makes the RC time constant directly correlate with the pulses. I always assumed it triggered at Vcc and 0V which I thought was pretty inconvenient since RC time constant only represents 2/3 charge or 1/3 discharge.

Well that's one thing where a PIC can't replace a 555 timer - high voltage applications.

Hero999

Yes, voltage doubling is one of my favourite uses for 555s.

Could you please post a schematic or is it top secret?

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dknguyen

As top secret as International Rectifier is.

I dunno, I just think charge pumps are so cool because it's one of those things where you're like

"It's pretty obvious. Charge up a capacitor to +V and then move it to somewhere else in the circuit and connect it's negative terminal to +V to get +2V."

Except that it was so cumbersome in my mind (I don't have access to all this custom silicon) to pull this off that it never occured to me that someone might pull this off in a wafer foundry. THat's just me. When I first started out in circuits the idea occured to me very early on, except I was like, "Naaaaahhh...too much switching going on. No one would ever do something like that." Of course, it had never occured to me at the time to use an RC-based timers rather than a crystal.

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Hero999

I tried to design and build a charge pump when I was about 12, from an astable multivibrator and some transistors, it would've worked too if I had known that you just can't connect two transistors bases together; I didn't realise that all I needed to do was add a resistor in series with each base.

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