[moved] Fault pin problems causing LED to light up for a short moment

[player80]

Hi,

I'm using a chip which has a fault pin (open drain), it sets the fault pin (which is pulled to 3.3v) low if a problem happens.
Now the problem is when I power up the circuit the pin is always set to low for a very short time until the chip is up and running.
I have connected a Mosfet to that pin which on the other side pulls 3.3v to ground during runtime as long as the gate pin is high (as long as no problem is detected the pin is high), however the startup is the problem since the gate will be low for a short moment.
This results that the LED will flicker for a short time when the circuit gets powered up.
Does anyone have an idea how I could delay the initial 0.5 seconds to avoid lightning up an LED when applying power to the circuit (without a microcontroller)?

- - - Updated - - -

A picture what the voltage ramp-up looks like at the fault pin

 

[E-design]

Re: Fault pin problems causing LED to light up for a short moment

This circuit shows the operation from 5 V, but you should be able to operate it from 3.3 V. Point VF1 will be the connection to the chip open drain connection. So input from VF1 is locked out until VF4 goes low. Time is set by the RC time constant. After the initial delay, the fault signal will appear on VF5 when generated.

 

[player80]

Re: Fault pin problems causing LED to light up for a short moment

This circuit shows the operation from 5 V, but you should be able to operate it from 3.3 V. Point VF1 will be the connection to the chip open drain connection. So input from VF1 is locked out until VF4 goes low. Time is set by the RC time constant. After the initial delay, the fault signal will appear on VF5 when generated.

thanks for updating it.

 

[player80]

I have changed it another way using a microcontroller. I'm using an NMOS as high side switch connected to the fault LED and pull the gate high after some time.