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LM74502H-Q1: Inrush current limit

Part Number: LM74502H-Q1
Other Parts Discussed in Thread: LM74502-Q1, LM74502EVM

Hello team,

My customer is using LM74502H-Q1 in their system, the load is around 300uF, the output voltage drops a lot which cannot meet their requirements.

1. Can LM74502H-Q1 used for soft-start (inrush current limit)? Or only LM74502-Q1 is more suitable for soft-start? Please help clarify and explain the detailed reasons.

2. If they use this way to prevent the voltage drop, is there any recommend Rg value and calculation tool to help evaluate this?

3. The driver capability is different between LM74502-Q1 and LM74502H-Q1, is there any other differences between two of them? A comparison table is better.

Thank you so much.

Best regards,

Lanxi Li

  • Hi Lanxi,

    For inrush current limit, we need slow-turn-ON so LM74502-Q1 with low gate drive strength would be good option.

    Only gate drive strength is different between LM74502-Q1 and LM74502H-Q1

    Use Rg in the range 100 Ohm to 220 Ohm and select Cdvdt as per equation (2) in the data sheet.

    Best Regards,

    Rakesh

  • Hello Rakesh,

    My customer use LM74502-Q1 sample to replace the LM74502H-Q1 in their original PCB, and they get an abnormal test results as below.

    It seems that the Vgate is quite strange which is following Vcap, I don't know why. Would you mind help check why this happen? Or give them some debug directions on these for your further investigation? It would be Okay if you can give me some rough idea about the root cause~

    Thank you so much.

    Best regards,

    Lanxi Li

  • Hi Lanxi,

    Can you share the schematic

    What is the Vin ? Can you increase CVCAP and check again.

    Please share better resolution test waveform 

    Best Regards,

    Rakesh

  • Hello Rakesh,

    SCH is as below:

    Vin is the battery, and they have verified that the test results are not related to the VIN voltage. Also, after they increase the CVCAP, there is still no improvement.

    If you need more detailed waveform, please tell me the detailed pin request for your investigation ASAP.

    Thank you so much.

    Best regards,

    Lanxi Li

  • Hi Lanxi Li,

    The Gate drive voltage is generated internally by the charge pump. As the gate of LM74502-Q1 is always fully enhanced when enabled, the GATE-SRC voltage will be close to VCAP-VS voltage. So, the ripple on gate voltage you are observing is expected. 

  • Hello GD,

    Do you mean of fully enhance of LM74502-Q1 gate? It would be best if you can give me some detailed explain about why this ripple? As you can see the waveform in the datasheet doesn't show the ripple?

    Thank you so much.

    Best regards,

    Lanxi Li

  • Hi Lanxi,

    Let me capture waveforms on EVM and get back to you with detailed explanation. 

  • Hello Praveen,

    Very appreciate that you can help with this if you are available, they need to know the reason and fix this solution in these 2 days.

    Thank you so much.

    Best regards,

    Lanxi Li

  • Hi Lanxi Li,

    The Gate driver of LM74502-Q1 is internally connected to VCAP by a PMOS internally when the gate driver is enabled. The PMOS resistance is very low in case of LM74502H-Q1 and hence has higher gate drive strength. So, you can expect the ripple on VCAP to be on seen on Gate signal also.

    Please find waveform captured on LM74502EVM. You can observe the VCAP ripple on Gate signal as well.

    The waveform in the datasheet below needs to be corrected. We will rectify this in the next datasheet revision. 

  • Hello Praveen,

    Thank you for the detailed reply about this which can help us understand.

    Add one more questions from my customer: Have you evaluated this voltage ripple internally, does it affect the drive of Mosfet?

    Thank you so much.

    Best regards,

    Lanxi Li

  • Hi Lanxi,

    The charge pump voltage  V(VCAP) – V(VS)) and FET gate-source voltage will be switching between 12.4V and 11.6V.

    Almost all the N-Ch Mosfet will be completely enhanced for gate-source voltages greater than 10V. Tthere will be no issue with ripple on FET gate voltage.