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TL2845: the delay time to output is above 300ns

Sammi Shang
Expert 4450 points
Part Number: TL2845

Dear team, 

Customer found in their test that the delay time to output is nearly 600ns which is above 300ns listed in datasheet. Please check the test waveform, the yellow one is the current waveform. 

Thanks.

Best regards,

Sammi

  • 0
    TI__Mastermind 31580 points
    Hi Sammi,

    What are the other two signals, is the blue one the gate drive?

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Expert 4450 points
    Hi Peter,

    Thanks for your reply. Yes, the blue one is the gate drive waveform.

    Best regards,
    Sammi
  • 0 in reply to Sammi Shang
    TI__Mastermind 31580 points
    Hi Sammi,

    I assume this is the voltage waveform on the CS pin ? If this signal is taken across the current sense resistor then any filtering RC circuit will delay the voltage signal on the CS pin.

    Note that the delay to output will depend on the over drive voltage on the comparator input and the exact DC threshold so there will be an apparent additional delay when the CS signal is ramping through 1V typical before the comparator switches then the delay to the gate drive output going low.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Expert 4450 points
    Hi Peter,

    Thanks for your explanation. So how will RC influence the delay time? If 300ns is still a solid indicator? Could you kindly give a more detailed description of this spec?

    Best regards,
    Sammi
  • 0 in reply to Sammi Shang
    TI__Mastermind 31580 points
    Hi Sammi,

    It is normal to add a series RC network from the current sense resistor to the current sense pin to filter out the leading edge spike that can occur on the current sense waveform.

    This RC circuit will add some delay in the current sense signal reaching the IC and will increase the delay from the current sense signal on the current sense resistor ramping up past the current limit threshold to the gate drive pin going low. Pleas also note that the use of a series gate drive resistor will also delay the gate drive signal from the IC pulling the gate pin of the mosfet low.

    Regards

    Peter