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LM7481-Q1: LM74810-Q1 charge pump issue

Carson Hsu
Prodigy 160 points
Part Number: LM7481-Q1
Other Parts Discussed in Thread: LM7480-Q1

I found that the Vcap-VS voltage is only 5.4V after the power is established, what are the reasons that the Charge pump voltage cannot be established?

  • 0
    TI__Guru 57737 points

    Hi Carson,

    The Charge pump voltage VCAP-VS should be around 12.2V - 13.2V. 

    To understand why the charge pump voltage is not as per specification, can you please provide us below information,

    1. Schematics
    2. Test setup and conditions
    3. Did the charge pump voltage measure 5.4 upon initial power up or is it after performing any tests? this is to understand if the LM7481-Q1 failed in the process of performing any tests.
    4. Have you tried replacing the IC with a new one ? Did it work ?
  • 0 in reply to Praveen GD
    Prodigy 160 points

    HI  Praveen GD

    How are you ?

    The reply is as follows.

    1.2. Schematics and test setup and conditions,please refer to the attachment.

    3. After power up measure the charge pump voltage.

    4.Not yet replaced with a new IC.

    LM74810.pptx

  • +1 in reply to Carson Hsu
    TI__Guru 57737 points

    Hi Carson,

    I am doing good. Hope you are good as well.

    Thanks for sharing the schematics. For me to completely review the schematics and provide comments please share the FET part number. Also are there decoupling caps close to the input of LM74810-Q1.

    In slide number that you shared, the only difference between LM7480-Q1 working sequence and non-working sequence is the 26V - 'Vout-local' rail. I don't see any obvious reason why LM74810-Q1 should not work when powered from PSFB Converter. Do you have any more observations.

    In the slide no. 3, the Gate voltage seems to be ramping up even before the input voltage started to ramp up. Are you switching the input power supply from 'DC Source 26V' to 'PSFB Converter' ? 

    Can you share the VCAP voltage voltage waveform as well during failure condition ?

  • 0 in reply to Praveen GD
    Prodigy 160 points

    HI  Praveen GD

    Thank you!

    1. FET part number : FDBL86561-F085.
    decoupling caps: Close to the pinA and pin VSNS? I removed the decoupling caps close to pin A and pin VSNS yesterday.

    2. In the slide no. 3.  Mainly the power of VOUT-LOCAL is provided by PSFB. Not provided by DC power supply.

    3. VACP waveform will be provided to you tomorrow, do you need other waveforms?

    4. UVLOF will lock the VACP voltage? How to check UVLO ?

    5. I tried to pull the OV pin low and EN/UVLO high(VS voltage). Doesn't seem to help.

    Carson Hsu

  • 0 in reply to Carson Hsu
    Prodigy 160 points

    HI  Praveen GD

    3. VACP waveform will be provided to you tomorrow, do you need other waveforms?

    The waveforms show as below.

  • +1 in reply to Carson Hsu
    TI__Guru 57737 points

    Hi Carson,

    There is lot of switching on DGATE because of which the charge pump can be stressed. This can happen at no load or very light load condition or during AC rectification.

    1. Are you testing at no load ? If yes, Can you please increase the load current to a level where the DGATE stops switching and then check the charge pump voltage ?
    2. Is there ripple on input voltage ? The DGATE keeps switching if there is a voltage difference (enough to trigger Vsd(rev)) between input and output voltage. 
  • +1 in reply to Praveen GD
    Prodigy 160 points

    HI  Praveen GD

    Now the function is working properly. We have not modified the peripheral circuit, and we need to follow up on the issue to avoid recurrence.

    1. Are you testing at no load ? If yes, Can you please increase the load current to a level where the DGATE stops switching and then check the charge pump voltage ?

           => No load test after loading DGATE on normal conduction

           2.Is there ripple on input voltage ? The DGATE keeps switching if there is a voltage difference (enough to trigger Vsd(rev)) between input and output voltage.

  • +1 in reply to Carson Hsu
    TI__Guru 57737 points

    Hi Carson,

    Ca you please elaborate on my below two questions,

    1. Are you testing at no load ? If yes, Can you please increase the load current to a level where the DGATE stops switching and then check the charge pump voltage ?
    2. Is there ripple on input voltage ? The DGATE keeps switching if there is a voltage difference (enough to trigger Vsd(rev)) between input and output voltage. 
  • +1 in reply to Praveen GD
    Prodigy 160 points

    HI  Praveen GD

    1. Are you testing at no load ? If yes, Can you please increase the load current to a level where the DGATE stops switching and then check the charge pump voltage ?

    ==> Yes,  no load. We have confirmed that the charge pump voltage is OK when we increase the load current to 10A.

    2. Is there ripple on input voltage ? The DGATE keeps switching if there is a voltage difference (enough to trigger Vsd(rev)) between input and output voltage.

    ==> We will confirm and get a reply back to you.

  • 0 in reply to Carson Hsu
    TI__Guru 57737 points

    Hi Carson,

    Thanks for confirming on point 1. Please go through the below e2e FAQ to understand more on why gate oscillates at no load.

    Ideal Diode Controller or Linear ORing Controller Gate voltage oscillates at light load or no load

    https://e2e.ti.com/support/power-management/f/power-management-forum/992989/faq-ideal-diode-controller-or-linear-oring-controller-gate-voltage-oscillates-at-light-load-or-no-load