LTC4368-1 Bi-direction Current and Voltage Protection Issue

To Analog Technical Support,

Que1 -> We are using LTC4368-1 IC for Bi-directional voltage and current protection voltage ranging from

UV=15V , OV=36V and Current -> -12.5A to +12.5A.

Que-2-> UV=3V , OV=5V and Current -> -500mA to +500mA.

Main issue seems to be that the GATE voltage does not get pulled low enough to turn-off the FETs when a reverse OC occurs. It seems to stay at approx 3V, so the FETs are not completely turned off. What is going on with the GATE voltage? 
Requirement - The main function for this circuit is the bi-directional over-current protection, to handle a short circuit on either side.
Suggest the solution for the same with reference.
Regards,
Pushkarsinh R
India
  • +1
    •  Analog Employees 
    on Mar 15, 2021 6:38 PM

    Can you provide a schematic and scope-shots?

    Waveforms of interest are:
    VIN
    VOUT
    GATE
    FAULT#
    Load current - using current probe
    VSNS - voltage across sense resistors using differential probe (if no current probe)

    Thanks,
    -Aaron

  • For Application 1: Attached LTSpce Simulation

    VIN: Max 36VDC, UV : 15V , OV : 36V
    VOUT: max 36VDC, I = -12.5A to +12.5A
    GATE-Attached LTSice Ckt
    FAULT#: Attached LTSice Ckt
    Load current - using current probe(LTSpice Simulation)
    VSNS - voltage across sense resistors using differential probe (if no current probe) -> 4mOhm , 1% . 2W for Current : -12.5A to +12.5A

    Ckt. simulation through LTSpice Tool

    Application1-LTSpiceWaveform1

    Application1-LTSpiceCkt

    For Application 2: Attached LTSpice Simulation:

    VIN: Max 5.5VDC, UV : 3V , OV : 5.5V
    VOUT: max 36VDC, I = -500mA to +500mA
    GATE-Attached LTSice Ckt
    FAULT#: Attached LTSice Ckt
    Load current - using current probe(LTSpice Simulation)
    VSNS - voltage across sense resistors using differential probe (if no current probe) -> 4mOhm , 1% . 2W for Current : -500m to 500mA

    Ckt. simulation through LTSpice Tool

    Application2-LTSpiceWaveform

    Application2-LTSpiceCkt

  • 0
    •  Analog Employees 
    on Mar 16, 2021 9:01 PM in reply to pushkarsinh.r

    Thank you for posting your sims. I can see a couple of issues:

    The first issue is that with the LTC4368, power should be on the VIN side, so you need to switch “left” and “right” on your schematic. This is because the VIN pin needs to reach VIN(UVLO) and the UV-OV window must be satisfied, or else the FETs will not turn on.

    Next, series diodes D1 and D2 should be removed. In cases where the supply goes negative, VIN is shorted to gate to turn off the FETs. If D1 is there, it will interfere with this mechanism, and the FETs will not be guaranteed to be off in a reverse supply situation.

    See the simulation below (set up like app 2):
    IN is ramped from -10V to 10V, and GATE stays off when IN is outside the UV-OV window. When the reverse circuit breaker threshold is tripped, the FETs turn off until OUT falls 100mV below VIN.

    4368-1 EZ 3-16-21.asc

    -Aaron

  • Also share suggestions for Application1 - UV = 15V,OV=36V, I = -12.5A to 12.5A