LT4356-3 operating during short circuit

Hi,

i would like to use the LT4356-3 as short circuit protection. During a short circuit event it can happen that the input voltage drops to a very low value due to high line impedances between source and LT4356-3. The voltage drop last only for a few µs until the maximum short circuit current comes up. This leads to a shutdown of the LT4356-3 and to a floating Gate potential. At least it seem like an uncontrolled gate situation. In this case I have measured to defects of the LT4356-3:

  1. Charge pump is only able to deliver 5 V on the Gate independent of the input voltage
  2. The sens Pin is not able to detect any current threshold

What is the problem here? Are there any design rules i should be aware off take to guarantee a save operation during short circuit without damaging the LT4356?

Thanks,

SCF

  • Hi  SCF ,

    During Short circuit , LTC4356 will connect Gate to GND as soon as the current limit circuit engages for longer than time out duration and GATE will not be floating. Did you measure the GATE voltage wrt IC ground or system ground as these might  be at different potential due to board layout and inductances.

    As for GATE drive voltage , it should be measured with respect to OUT pin voltage. At Vcc>8V , the minimum gate drive is 10V and max. is 16V. However during regulation the gate drive will be less to maintain the sense voltage. If Vcc is =4V gate drive can go as low as 4.5V min. If you are seeing gate drive of 5V even at higher input voltage during normal conditions , I would recommend checking board layout for any leakages from GATE pin.

    You can send us the waveforms showing Gate , Output  and differential sense voltage along with Timer pin voltage so that we can better explain what might be happening during the short circuit condition. Also, please send the schematic so that we can check if there is any  reason for this  abnormal behavior.  

    Best Regards,
    Ritesh