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DRV110: Out pin duty cycle less than 1%

D Witte
Prodigy 10 points
Part Number: DRV110
Other Parts Discussed in Thread: UCC37322

I'm using the DRV110 in a design to drive a 120VDC solenoid in Peak and Hold operation.

When I increase the I_hold set point, the OUT gives me something that I don't expect: A long pulse followed by many very short pulses (less than 1 us) at 20kHz.

Is there a reason for this duty cycle of less than 1%  It does not seem to agree with the datasheet.

In the pictures below, yellow is OUT and Green is the voltage between the solenoid and Mosfet as shown in the schematic below

  • 0
    TI__Guru 56650 points

    Hello, D Witte,

    Would you measure the sense pin voltage and put it on the same scope picture with OUTx to help us understand the issue?

    Regards,

    Wang Li

  • 0 in reply to Wang5577
    Prodigy 10 points

    Hi Wang,  

    In the pic below,

    YELLOW is OUT

    GREEN is SENSE

    RED is current thru Ls (differentially measured across a .25 ohm resistor in series with Ls)

    The R_hold value is 245 kohm to give a Vref of .040 mV

    The R_osc is 0 to give 20kHz PWM frequency.

    Rsense is .015 ohm

    Vs is 120VDC

    I'm using the EVAL board and have the MOSFET as shown in the previous post mounted externally.

    Thanks for your assistance!

    Drew

  • 0 in reply to D Witte
    TI__Guru 56650 points

    Hello,

    It seems the current sense circuit filter (R_filt and C_filt) cannot get rid of the switching noise which could trigger the DRV110 current regulation comparator. 

    Possible reasons to have a strong switching noise:

    1. There is no local ceramic capacitor between AM1 point to R_sense ground. Please try to add 0.1uF ceramic capacitor to see if any output waveform change.

    2. UCC37322 driver may be too strong and cause the noise. Since the switching frequency is only 20kHz, the switching loss may not be very high. Would you short out UCC37322 to use DRV110 driving the power FET, T3, directly?

    3. Would you increase the C_filt to 1nF to try a strong filter to see if it can filter the noise out?

    Regards,

    Wang Li 

  • 0 in reply to Wang5577
    Prodigy 10 points

    Hi Wang, Increasing the C_filt helped quite a bit.  Thank you

  • 0 in reply to D Witte
    TI__Guru 56650 points

    Hello, Drew,

    BTW, UCC37322's gate drive current goes through the sense resistor according to the its ground and FET source/ground connection. The strong gate driver can cause a strong noise on the sense resistor.

    Regards,
    Wang Li