Pic driven capacitor doubler for 5.5v voltage, from 40v with no drop-out

Hi,

I have an unusual requirement which I'd have resolved with a sepic if I could find one with the range. Basically I have a PIC which controls a small FET. The FET has a relatively high drive voltage and there's a capacitor decoupler in front of the gate to isolate it from the load which drops a bit of voltage. From 5V into the PIC I get about 3.5v at the gate. RGS is 4V for reasonable resistance..

The input voltage is somewhere between 40v to 5v. 5V is the absolute minimum for the PIC to be able to drive the gate, which means using a zener/transistor follower drops 0.7v and is a no-go. Same for any sort of LDO, not that I can find anything decent which handles the input range.

Current is low, no more than 0.1A. My first thought was to use a a 3.3v LDO into a buck. My second, which I've attached a diagram off is to use two LDO's and the PIC to drive a voltage doubler. Any other suggestions?

Cheers, Andrew

Depends on what it is, heres 1 idea, build a shunt reg, series R and zener across the pics rails, then dropout is very low, this will work if theres nothing but the pic to power, and for gate drive you could use an inductor a diode, cap and zener as a primitive step up reg, this will be a little slow in pic terms to build up enough V to turn on the fet.
And you can get low threshold power fets, 2v is realistic.

I estimate current to be 40mA peak at 5V, which is a 125ohm resistor. But at 35.1V that's 270mA of wastage and a 9.5W resistor.

Can't change the FETs, I'm space critical and the particular model I use is perfect other than the 4v turn on.

The boost is a good idea but I have 3 fet's to drive and the extra inductors and diodes would be problematic and certainly would have to be carried out without regulation. Speed isn't too critical (and I have looked at isolated LED drivers like the VOM1271 but the cost and footprint size makes it a non starter.

Theres a web article on driving white leds from 3.3v microcontollers using the 2 o/p transistors to chartge & synchronously discharge an inductor to boost the voltage, you could adapt this, but I cant find it now.
You'll need a zener to protect the gate of the fet, unless it has one built in that will take the current.