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Original question:

TPS65310A-Q1: The reasons about TPS65310A_Q1 start-up fail

TPS65310A-Q1: Vbuck3 fail to start due to high ripple

Clement Magnien
Prodigy 90 points
Part Number: TPS65310A-Q1

Hello,

Vbuck 3 (1.2V) seems to be instable: when reaching 1.2V after 1ms there is a big ripple (200ms) and after1.2ms the chip stop to work( P fet activated). I suspect it's the UVLO and OVLO on the buck 3 due to the high ripple. Buck1 (6V) and buck2(5V) are clean and the regulation works  well. Unfortunately there is only one control loop to set for buck1. Comp 3 is set to GND (3.3uH,30uF, 1.2V). Any idea how to fix it?

Thanks

  • 0
    TI__Expert 8730 points

    Hello,

    What is your inductor value and output capacitor values? If it is related to compensation, then increasing the output capacitor should make the BUCK stable. It could also be possible that regulator is overloaded and hitting the current limit. If you send some oscilloscope plots showing switch pin waveform, output waveform and your schematics, i can take a look at it. Is this happening on every device/board or issue shows up on only one two devices/boards?

    Regards,

    Murthy

  • 0 in reply to Krishnamurthy Hegde
    Prodigy 90 points

    Hello,

    Buck3 : inductor value=3.3uH, output capa= 30uF, Vout=1.2V, no load, I try with 10mA and 100mA, same results.

    I increased a lot the output capacitor  but I still have the problem.

    I connected Vmon3 to Vmon1 to deactivate the buck3 to see if other  rails would start correctly but Buck2 doesn't start???( should deactivate only buck3).

    All my board behave in the same way.

    Regards,

  • 0 in reply to Clement Magnien
    TI__Expert 8730 points

    Hello, can you remove R97 and see whether BUCK3 comes up fine? If regulator is not overloaded and increasing output capacitor does not help to get rid of the issue, then issue may be related to the layout related coupling. Have you verified whether input supply at VSUP3 pin is clean? You can try to add some 1nF capacitor on VSENSE3 to GND see whether it makes any difference?

    BUCK2 should work fine if you have connected VMON3 to VMON1 and remove R69 VSUP3. If BUCK3 works with noise, then it might also affect other regulators and cause some trouble. from the scope plots you sent, i cannot make out anything, other than seeing some ringing/oscillation on output.

  • 0 in reply to Clement Magnien
    TI__Expert 8730 points

    Hello, can you remove R97 and see whether BUCK3 comes up fine? If regulator is not overloaded and increasing output capacitor does not help to get rid of the issue, then issue may be related to the layout related coupling. Have you verified whether input supply at VSUP3 pin is clean? You can try to add some 1nF capacitor on VSENSE3 to GND see whether it makes any difference?

    BUCK2 should work fine if you have connected VMON3 to VMON1 and remove R69 VSUP3. If BUCK3 works with noise, then it might also affect other regulators and cause some trouble. from the scope plots you sent, i cannot make out anything, other than seeing some ringing/oscillation on output.

  • 0 in reply to Krishnamurthy Hegde
    Prodigy 90 points

    Hello,

    R97 is not connected, I load it with a resistance in parallel. with no load same problem.

    I added 100uF in output, it's better but the Ic detect a fault and stop all the rail.

    Vsup3 is clean but start to have ripple when 1.2V starts and oscillate.

    I tried 1nF across Vmon3 and gnd, same result.

    I disconnected the 1.2V ( not supplied R74 off, Vmon3 connected to Vmon1).  the 5V is clean but now the 6V oscillate and it stops after 2ms(see picture)

    What could make the IC to stop Buck1 and buck2 when buck 3 is deactivated?

    regards,

  • 0 in reply to Clement Magnien
    TI__Expert 8730 points

    Hello,

    If you have correctly connected VMON3 to VMON1, if you have disconnected VMON3 from BUCK3 output and if you have disconnected BUCK3 input supply from VSUP3, PMIC should work fine, unless something else is causing the PMIC shutdown. Please refer to the device datasheet for possible causes of reset. Please share complete schematics and layout diagrams so that I can take a look at it completely.

    adding / removing capacitor on VMON3 wont help to filter the noise as VMON3 is used only for monitoring and not for regulation. As i already said, if one rail starts to oscillate, there is lot of noise in the board/GND and it can affect all the regulators. 

    Regards,

    Murthy