Considerations for designing a PI

[Bill]

I had a couple of quick questions regarding general PI design. How detrimental is it to have a power supply circuit (like a charge pump) that is not clocked by the main transmit signal? I have seen some nice doubler/inverter chips that use internal 1mhz oscillators.

At the low frequenices that we use with PIs is it worth using a negative voltage on the mosfet gate to discharge the gate capacitance faster? I have seen some mosfet assisted turn of schemas that use a inverted signal realtive to the gate drive signal to force the gate to Gnd. Does the added input capacitance of such a setup effect the overall speed of the coil?

[bugwhiskers]

Hi Bill,

Do a forum search here for "TC426". It's a MOSFET driver that can switch 1000pF in 30nS. It has 2 gates so the second one can be used for a charge pump, plus or minus.
In the next couple of days I will be posting schematics for 3 circuit boards that make up the UPIM that uses the chip as described. They will be in Tech Forum\Universal PI Micro.

regards
bugwhiskers

[Carl-NC]

In the HH project I use a 7660 charge pump chip. In the first rev I let it free-run, and everything seemed to work fine. In the second rev, I fed the 7660 a sync off the main clock. I never saw a difference with/without the sync, but that was indoor testing where other RF noises could easily have swamped the effect. In general, I think you should sync the charge pump.

- Carl

[Bill]

Thanks guys! I am in the middle of designing a PI for learning purposes. And all your input is very helpful. Here is the front end schematic. I am just wondering does the added input capacitance (gate capacitance of the turn off fet ) effect the coils speed.