Driving a FET via a micro

[fxdif]

Hi, I'm trying to drive a FET via a micro with PWM (ST7FLITE09).

I'm doing this via a 22R resistor from Port pin to gate. However when I do this the voltage from the micro drops to about 1V and won't turn on the transistor. Any ideas anyone as I am stuck???

 

[lcsjk]

REad the data sheets! St7 cannot drive 25mA at a level to turn on/off the mosfet at high speed, and indeed without a limiting resistor you may violate absolute specs. Make sure you have a limiting resistor on the output pin to limit the current to its max level though (not less than 200 ohms for 5V at 25 mA).
Then do as Melone suggested and hook a 10k resistor to the UP output and observe with a scope (not a DVM) to see it the waveform is going high and low properly. Finally, make sure your FET is switching on/off fast enough by observing with a scope. Your present buffer PNP may be enough with the 1k pulldown on the GS of the FET, but make sure your FET drain-source is looking like you expect.

 

[Astrax]

>Would it make any difference that I am using the FET to switch an inductive load (the circuit is part of a boost converter)?

Inductive load is a nasty one. Have same problem here in a echosounder Tx-powerunit drive (6A at 100V). The FET gate source must be low imp. to drive the FET. Good drive improve switching time - reduce power loss -
Take a look at irf an-937.pdf

rgds

AJK

 

[felixc]

Before questioning the FET interface or drive problem, I think you should first remove all that circuitry, expect maybe a pullup. Does any PWM signal gets out of your micro?
Felix

 

[zeitghost]

What eventually permeated my brain was that the IRLZ34 has a zener protected gate.

Sometimes the zener gets damaged, which increases the amount of current required to turn the "voltage operated" FET on.

Just a thought.

rgds
Zeit.