Have designed the circuit below. A press on SET should turn LP0701 on & deliver power to the rest of the circuit. ICM7242 runs & active (Low) output connected to Gate holds FET on until RESET is pressed or timer expires. Timer output should then switch to high and turn off FET.

However, when timer output returns to high the voltage on the Gate only goes up to 1.89V so FET doesn't turn off. If I disconnect the timer output from the Gate it goes to 3.6V and Gate goes to 0V.

If I put my multi-meter (on 2mA setting) between timer output and FET Gate it shows 0.00mA so, as we'd expect, it's not that the FET is demanding more current than the timer output can supply.

Can anyone suggest why the Gate voltage doesn't go to 3.6V when timer output is high?

 

What is terminal 34 connected to? What is the load?
SG

 

Thanks for input.

With 1K the Gate voltage goes up to 3.22 (currently on 3.3V supply) but only down to 1.99. Voltage on FET Drain 1.35V and 2.58V so looks like getting neither fully on nor fully off?

 

What is terminal 34 connected to? What is the load?
SG

Presently there is no load. The proposed load is an input to a telemetry transmitter which I recall has 20K pullups on its inputs.

 

Something else doesn't look right, why is pin 5 connected to the output>
SG

 

Something else doesn't look right, why is pin 5 connected to the output>
SG

To make the timer work as a monostable pin 5 (Reset) is connected to pin 3 (Output).

 

To make the timer work as a monostable pin 5

OK. Try a 10K from gate to supply.
SG

 

OK. Try a 10K from gate to supply.
SG

Tried that after 1K Gate voltage goes to 2.6V Drain goes to 1.98 & nothing runs....

 

Sure you have the FET installed correctly?
SG

 

Pretty sure - I've never used them before before but I've checked the data sheet and it looks right to me.

 

Sure you have the FET installed correctly?
SG

Source to 3.3V supply, Drain to rest of circuit.

 

The mosfet is backwards. The source should connect to 3.6v supply.

The IC needs time to stabilize before trigger input signal will work (setup time).

 

Something seems off with the ICM7242 then. Data sheet shows output should be below .1 volt with 10K load at 3.6 supply.
SG

 

Something seems off with the ICM7242 then. Data sheet shows output should below .1 volt with 10K load at 3.6 supply.
SG

The output is normally high and goes low when triggered and timing out.

 

Correct but output should be below .1 volt when low. Double check mosfet your schematic shows it upside down.
SG

 

Correct but output should be below .1 volt when low. Double check mosfet your schematic shows it upside down.
SG

Have connect output to 0V via 10K and voltage is 0.00V.

 

The output is normally high and goes low when triggered and timing out.

Goes Low for 128 cycles of RC and then high for 128 in Astable mode but in monostable the return to High resets timer connection to pin 5.

 

Have connect output to 0V via 10K and voltage is 0.00V.

Connect 10K from output to supply and read when low, disconnect FET.
SG

 

Goes Low for 128 cycles of RC and then high for 128 in Astable mode but in monostable the return to High resets timer connection to pin 5.

Yes...but there is a race condition between trigger and power-up. The trigger happens too fast.

 

Connect 10K from output to supply and read when low, disconnect FET.
SG

Will do. FYI when low the output happily drives >11mA into an LED connected to supply.