I have a project where im pwming a mosfet @20khz with a attiny85, the prototype used a 10k pulldown resistor because that's usual practice, but im wondering how necessary is that? I mean the gate will be driven high and low constantly while power is applied, and the only time there will be power without switching is after a pushbutton, which will just set the output constant low, so it will stay driven, the only floating time would be when power is first applied until the outputs set up
adding a 10k pulldown to my pcb wouldn't be that hard except I already submitted it to batchpcb and made it extremely compact and ill have to make it a little bigger to fit two 0805 resistors, which im wondering is it worth the effort?
Also this is for dimming leds so if they flash once while the outputs are still floating it wont matter especially since I imagine the outputs will set up much faster than visible
When a mosfet gate is being driven by a active push/pull output (active sinking and sourcing of current) signal like an AVR output develops the mosfet does not require a gate to ground pull-down resistor to function. It's main purpose is as a safety issue if there was ever a time where the mosfet drain circuit voltage powered up, but the AVR device driving the mosfet is powered off, the pull-down resistor would force the mosfet to an off condition.
Lefty
winner10920:
I have a project where im pwming a mosfet @20khz with a attiny85, the prototype used a 10k pulldown resistor because that's usual practice, but im wondering how necessary is that? I mean the gate will be driven high and low constantly while power is applied, and the only time there will be power without switching is after a pushbutton, which will just set the output constant low, so it will stay driven, the only floating time would be when power is first applied until the outputs set up
adding a 10k pulldown to my pcb wouldn't be that hard except I already submitted it to batchpcb and made it extremely compact and ill have to make it a little bigger to fit two 0805 resistors, which im wondering is it worth the effort?
I would use a pulldown because even though you have full totem pole drive when the processor is POWERED, you have basically an open circuit when it's not powered.
Now, the processor may have it's own ESD protection diodes internally and provide protection for the gates, but you can't count on that.
The processor is powered from the same supply as the leds, so the leds will never have power without the processor
what would those esd protection diodes do?
what would those esd protection diodes do?
Protect against ESD.
As was said your only problem is the time between reset or applying power to the initialization of that pin as an output. The worst that could happen is that the thing it is powering could briefly flash on during power up.
the processor may have it's own ESD protection diodes internally and provide protection for the gates, but you can't count on that.
It has, and I would.
Ok, so ill probably go ahead and leave it without the pulldowns, I don't mind if there's a flash, thanks for the advice
which im wondering is it worth the effort?
It depends on your driving circuitry and generally is not critical: most of the times you are driving a n-channel mosfet with a totem output stage or a open drain/collector output stage. Both types of circuits short the gate to ground when they go low, negating any benefits from a pull-down mosfet.
Pull-up mosfets is another question, for open drain / collector drivers.