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ISO5500 problem

Shrikant Lokhande
Intellectual 655 points

Hello All,

I have been using ISO5500 IGBT/MOSFET driver IC for one of my switching operation.(MOSFET used is IRFP4668)

I have also used the same driver for H bidge circuit of same MOSFETs for current reversal.

IC has input power supply of +5V and output power supply of isolated +15V.

While trials I observed that the VIN+ (pin no 1) when kept open shows 5V and Vout also shows +15V , I tried it with 10K pull down resistor but it shows same voltages.

Correct me if m wrong but it may fire my MOSFET when the  Vin pin is open.

Please guide me as this unit will shortly come for the production.

Thanks and regards,

Shrikant L.

  • 0
    TI__Genius 14015 points

    Hi Shrikant,

    Can you share your schematic with us? That will help us determine what is going on. 

    Thanks!
    Jason Blackman 

  • 0 in reply to Jason Blackman
    Intellectual 655 points

    Hi Jason,

    Sorry for being late on this.

    Above is the schematic of my ISO5500 test setup in non-inverting config.

    In above ckt DESAT pin is open but actually I have given it to Drain of my MOSFET through a series resistance of 100E and BYV26E diode as shown in datasheet.

    My ckt is not in auto reset mode but it can be configured for autoreset using a short link between Vin+ and /RESET.

    The pull down resistor is of 10K value between pin 1 and pin 2.

    I have exact arrangement as shown in datasheet but still I see 5V on Vin+ (pin1) when kept open.

    Please explain me what would be the reason for this.

    Thanks and Regards,

    Shrikant L.

  • 0 in reply to Shrikant Lokhande
    TI__Genius 14015 points

    Hi Shrikant,

    There is a very weak pull-up on Vin+ (~700kΩ) internal to the device so that the inputs are in a determined state when floating. That being said, the 10kΩ pull-down you are applying should be more than enough to get the job done. 

    I suspect that the microcontroller or something else on the board is driving that pin high. When you say it is kept open, what do you mean exactly? Is the microcontroller output in hi-z state, or are you removing R11? 

    Let me know. Thanks!

    Jason Blackman 

  • 0
    Prodigy 100 points

    1st how do you post a new question?.

    2nd in this case the data sheet says weak pull ups should be avoided. I use a totem pole drive to the input pins.

  • 0 in reply to Stephen Brown2
    TI__Intellectual 2200 points

    Hi Stephen,

    There should be a red button that says "New Post" to the right of the page when you enter the forum. Else, could you try clicking on the link "Forum" to your left, and then you should see the "New Post" button to your right?

    The idea of driving the inputs with a strong buffer (such as CMOS) is to reduce the impedance on the input pin. If you use a totem pole with a pull up, you could see issues due to noise coupling from the switching high voltage side to the input pins. The extent of the problem will depend on the rate of switching as well as the value of parasitic capacitance from the high voltage side to the low voltage side in your system.

    Rgds,

    Anant