Charge Pump Shoot Through Resistor

I'm making a charge pump to provide a floating supply (to replenish the boostrap capacitor for the high drive MOSFET so it can maintain continuous gate drive).

I can use a 555 timer or some isolated charge pump circuit IC from ONSemi, but they all require a few too many components and I'm running out of room and need a few of these lying around the PCB. I was thinking I can cut out the timing capacitors and Vdd/Vss pins normally required on such ICs if I just use a CMOS pair and use a pin from the uC as the clock instead (please see diagram).

I only have one pin to implement this on (no software controlled dead-time), and it would somewhat defeat the purpose of minimizing components for this anyways if I used two pins. It would also very obviously defeat the purpose if I used gate driver ICs with dead time. So, I was thinking I could just place a resistor on the source of the PMOS to deal with shootthrough. I don't think any current realy flows through the PMOS- it's just used to float the capacitor to charge the floating cap.

Does anyone see a problem with this?

EDIT: Oh wait, the current that transfers charge between the caps passes through the resistor. Hmmm. ANd splitting the resistor between high and low side ends up speeding up the charge transfer a bit while slowing down the charging of the capacitor that gets boosted up. Does anyone know of a realy simple way to get around this? Maybe I should just size the resistor appropriately?

How do 555 timers deal with shoot through in their push-pull circuits? I'm pretty sure they don't have anything like that but I could be wrong.

I'm not sure what you're trying to do, but here's the things to keep in mind:
1. The NMOS and PMOS transistors have an internal diode
on the drain-source which is reverse-biased in the "normal" direction. Any time the Vds is substantially (>0.3V to 0.7V) negative in an NMOS or
positivie in a PMOS, this diode will conduct.
2. The flying cap must form a complete circuit to charge or discharge. I am unsure of the completeness of this circuit as the nature of V_float is not adequately depicted.

Sorry, didnt notice that it wasn't clear. Vfloat=V in my case.

Yeah, I was thinking about problem about the diodes too (though when thinking it through I didn't seem to see any times they might conduct). BUt these diode problems along with shoot thorugh, don't they also exist for the push-pull in a 555 timer? I've never heard of anyone running into these problems with a 555 timer.

This is meant to try and replace the 555 timer charge pump on page 18:
https://www.electro-tech-online.com/custompdfs/2007/12/an-978.pdf
I'm basically trying to get rid of the timing resistor and capacitor.

EDIT: Well, it seems that whatever I do to try and get rid of shoot through will need more parts than just using a 555 timer and someting as simple as resistors brings it down to the current capacity of the 555 timer in the first place. So unless I can ignore shoot through for some reason, I guess I'll go with the good old 555.

I don't understand why you need more than one floating supply and why you need bootstrap capacitors if you're using floating power supplies.

Suppose you have a simple NMOS h-bridge, it's only the high side transistors that need a gate drive higher than the supply. You only need bootstrap capacitors when you don't have another higher voltage power supply handy and it saves having to bother with any fancy switches to drive their gates from the higher voltage.

How much current do you need? You could use the output from the uC directly if the current is low. Also, that circuit won't work if V is at a higher potential than the uC's supply rail because the PFET will always be on. You could use a bipolar arrangement for more current, but the output swing would be 1.4V lower:

It's okay, thanks. I Just realized I can't ues a uC for the clock because the whole thing (as shown in page 18 of the app note I posted earlier) has the negative supply of the whole charge pump is referenced to the motor battery voltage (rather than the positive supply being referenced to ground)- that's how the whole thing is able to provide a 13-15V across the gate-source of the high-side MOSFET no matter where it's floating. Since my uC clock is referenced to ground, it just ain't gonna work. Score one more for a 555 timer.

Hero999 said:

I don't understand why you need more than one floating supply and why you need bootstrap capacitors if you're using floating power supplies.

Suppose you have a simple NMOS h-bridge, it's only the high side transistors that need a gate drive higher than the supply. You only need bootstrap capacitors when you don't have another higher voltage power supply handy and it saves having to bother with any fancy switches to drive their gates from the higher voltage.

Click to expand...

I don't have a floating supply. THe charge pump IS my floating supply.. I'm sort of "floating the entire charge pump circuit" so it can refresh the bootstrap capacitors to allow for continuous gate drive.

An ordinary 555 has a max output current of 200mA. Its output shoot-through current is a spike of 400mA which is why a 1uf and a 0.1uF capacitors are recommended as supply bypass capacitors for it.

A Cmos 555 has a much lower max output current and a correspondingly low shoot-through current.

74HCxx high-speed Cmos logic circuits have an output shoot-through current of about 50mA. Therefore they need a supply bypass capacitor at each IC and have a max amount of time they can be linear (a minimum allowed input pulse rise-time).

audioguru said:

An ordinary 555 has a max output current of 200mA. Its output shoot-through current is a spike of 400mA which is why a 1uf and a 0.1uF capacitors are recommended as supply bypass capacitors for it.

A Cmos 555 has a much lower max output current and a correspondingly low shoot-through current.

74HCxx high-speed Cmos logic circuits have an output shoot-through current of about 50mA. Therefore they need a supply bypass capacitor at each IC and have a max amount of time they can be linear (a minimum allowed input pulse rise-time).

Click to expand...

Ah, I see. I didn't know that how the capacitor was how they dealt with shoot through. Neat.