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CSD97396Q4M: the mosfet can not be opened

Part Number: CSD97396Q4M

Dear sir,

I'm using the CSD97396 to implement a buck-boost circuit. The PWM and skip is connected to a FPGA. They are 3.3V signal. The PWM frequency is 400KHz. No matter I change the signal, there is no voltage output. I cannot get any voltage on SW pin of U17 even I set pwm = 1.  The Vin is 12V in my test. R169 and R170 change to 1ohm.

Could you help to check what is the problem?

Thanks

BR

Ren

  • BR,
    I am looking into this, hopefully I will have a response for you in the next 24 hours.

    Best Regards!

    Brett Barr

    Product Marketing Engineer

  • In reply to Brett Barr1:

    What is your bootstrap voltage? If this is too low, the HS cannot be turned on.

    Best Regards!

    Brett Barr

    Product Marketing Engineer

  • In reply to Brett Barr1:

    Hi Brett Barr,

    I used the multimeter measure the voltage on C147 and R169. The voltage of BOOT pin is 4.9V, the voltage of BOOTR pin is close to 0.

    Is there any problem?

    Thansk

    BR 

    Ren

  • In reply to HongRen Chang:

    My current setting is: The PWM2_DPWR, SKIP1_DPWR and SKIP2_DPWR are always high, the PWM1_DPWR is the 400Hz PWM signal input.
  • In reply to HongRen Chang:

    Sorry for mistake, the PWM is 400KHz signal. I adjust its frequency to 100KHz and change the different duty cycle. It is still no output.
  • In reply to HongRen Chang:

    Can you check the SKIP# logic state at power up to make sure it hasn't entered sleep mode. If SKIP# is in tri-state at power up, it will need to go low for at least 100us to activate the device before going high.

    If SKIP# is high and boot strap is also high enough, we're a bit at a loss; the next step would be to check the solder/connections to make sure all the pins are connected.

    Best Regards!

    Brett Barr

    Product Marketing Engineer

  • In reply to Brett Barr1:

    Hi,Brett Barr

    Thanks a lot.

    It works when I set skip low first and then go to high.

    But I found a strange thing.

    1. When I set the circuit worked under buck mode. skip1=1,and pwm1 = pwm_sig. skip2=1 and pwm2 = 1. The buck side works well. I meaused the voltage on the inductor, it changes as expected. The U18 should worked under pass through mode as I expected, but the voltage of Vin on U18 is the double voltage of  the inductor .

    2. When I set the circuit worked under boost mode. skip = 1 and pwm1= 1, skip2 = 1 and pwm2 = pwm_sig. The high side mosfet of U17 seems not opened fully. The input voltage 12V, the voltage on inductor is only 0.4V. the U18 seems worked, the output signal is changed with different pwm frequency.

    It seems the device worked strangely when PWM is always 1. What is the problem? 

    By the way, Is there any limit for the output for chip? 0~24V?

    Thanks

    BR

    Ren

     

  • In reply to HongRen Chang:

    Ren,
    Sorry for the delayed response. When PWM is always held high, the bootstrap will not have any time to charge driver IC to provide the gate voltage to turn on the high side FET. So that would be why you are seeing the behavior you are seeing.

    As for the max Vout, the max voltage across the FETs inside the device should never exceed the recommended Vin (24V) on the datasheet.

    Best Regards!

    Brett Barr

    Product Marketing Engineer

  • In reply to Brett Barr1:

    Dear Brett Barr,
    Thanks for your reply.

    BR
    Ren

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