LT4363 Gate Pin Voltage Issue


I am designing an overvoltage and reverse polarity protection circuit using the LT4363 Surge protection chip. The circuit I am designing is intended to protect our system against overvoltage spikes and also against reverse polarity conditions working for input voltage levels of 10-48V and a current up to 5A. the designed circuit, which makes use of the LT4363 under and overvoltage clamping features is shown below.

I have tested this circuit on LTSpice and upon confirming its functionality and have moved on to breadboard prototyping, however, when implementing the circuit on a breadboard, the chip seems to display unexpected behaviour. Specifically, when the attached circuit has been implemented, the gate pin seems to only output a maximum of 9V, even when 48V have been supplied to the Vcc pin and the voltage drop across the forward N-MOS is 40V, thus the voltage at the output is only 6V, the load in this case is an LED connected through a 47kOhm resistor, however the issue persists when using the circuit to power a load with higher current draws. The subsequent gate to source voltage for M1 is only around 2V in each case however. I am suspecting that the voltage drop is due to the MOSFET not turning on fully and this displaying a high channel resistance, but as the chip is specifically designed to function as a high side MOSFET driver, I am unsure why it is not turning on the MOSFET fully. Would this be due to an issue with my designed circuit or is it an issue with the LT4363 chip? Furthermore, as I am attempting to operate the chip at currents of around 2-3A with a maximum of 5A through the NMOS, would this particular chip be able to handle such current draws? Thank you.

  • 0
    •  Analog Employees 
    on Sep 9, 2019 9:40 PM

    Hello ABJ,  The LT4363 can operate with 48V and normal gate voltage should be around 61V (48V + 13V typical dVgate). Please double-check the following:

    - Check that the LT4363 is not already damaged. 1N5378B is a 100V Zener, which is not going to protect the LT4363 100V VCC absolute maximum rating. Use a Zener whose breakdown is well below 100V but above your 48V operation.

    - Check that the MOSFET is not already damaged. Limit current through the MOSFET when GATE is stuck low since that will cause excessive DC power dissipation in the MOSFET (40Vds X Ids is 40W at 1A), damaging it. Disconnect M1 and M2 from LT4363 to check if GATE goes high.

    - Check that there is no unexpected board leakage that is holding up GATE from rising. GATE is pulled up with 20uA minimum so leakage should be lower than this.

    - A 22uF capacitor is needed on output per datasheet page 16.

    - IRFZ44N BVdss is 55V, too close to 48V operation. It should be higher than your maximum expected input surge (datasheet page 13).

    For further questions specific to your application, please submit a Technical Support Request: