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LM63635-Q1: Vout problem on standby mode with power off timing

Satoshi
Guru 19495 points
Part Number: LM63635-Q1

When LM63635-Q1 power(VIN) off, standby mode occur Vout switching and keep about 2.4V. 

(External setting on Voff: 6.45V_typ)

Please see attached waveform below;

※ch4：Inductor current

Please let me know about two points below;

①What is the reason for Vout switching and keep 2.4V on standby mode?

　Is these waveform abnormal, please let me know idea for improve point. 

②Is there possibility to reason for low Cboot value?

LM63635-Q1's circuit information 

・24Vin → 5Vout / 3A

　Vout is adjustable setting, VSEL connecting 10kΩ

・Cin: 10μF ×2, 0.22μF

・Cin: 22μF ×3, L:2.2μH

・Cboot: 220nF / 50V (maybe decrease by DC bias)

・fsw: 2.1MHz

・FPWM mode

・External UVLO setting

　Von: 10.34V_typ, Voff: 6.45V_typ

・/RESET is floating (unused)

Best regards,

Satoshi

  • 0
    TI__Guru 57555 points

    Hello

    Can you share the schematic of your DC/DC application.

    Also, can you show the EN and VCC voltages on the scope.

    You can remove the SW and inductor current if you don't have enough channels.

    Thanks

  • 0 in reply to Frank De Stasi
    Guru 19495 points

    Hi Frank,

    Thank you for reply,

    I attached schematic and scope with EN and VCC below;

    Additional information, When Vin lower than VEN_L, Vsw occur to continue small switching.

    Additional question;

    ・Are internal FET and driver not disable on standby mode by any condition?

    ・Is datasheet 8.3.6 (page 17~18) relate to this problem?

    Best regards,

    Satoshi

  • +1 in reply to Satoshi
    TI__Guru 57555 points

    Hello

    When the EN falls below the EN threshold or the input falls below the UVLO level, the device will shut off.

    What kind of load are you using ??  Is it constant current or constant resistance ??

    It may be that your load is some how overdriving the output.

    Try to disconnect the customer load and use a resistance and test again.

    You may also want to test this on an EVM with a resistive load and then test with the customer load.

    Thanks

  • 0 in reply to Frank De Stasi
    Guru 19495 points

    Hi Frank,

    Thank you for advice,

    Load is customer's real circuit board, with connecting many circuits.

    When load is changed to resistance, Vout is clearly shutdown. 

    I understood affect from overdriving the output side, is there any idea for clearly shutdown on real circuit board condition?

    By the way, datasheet P18, 8.3.6 is described about discharge function.

    Is discharge function not to improve points?

    Best regards,

    Satoshi

  • +1 in reply to Satoshi
    TI__Guru 57555 points

    Hello

    The pull down will help to bring down the output voltage by discharging the output capacitor.

    However, it appears that something in the customer load is causing an issue.

    You should first try to disconnect the customer load and replace with a resistive load.

    If that solves the issue, then you can replace the loads, one-by-one and see which one is causing the issue.

    Thanks

  • 0 in reply to Frank De Stasi
    Guru 19495 points

    Hi Frank,

    Sorry, output side is customer's board and could not test one-by one and check which one.

    Please see additional question below;

    ①The waveform for inductor current feel like to start discharge on lower VEN_VCC.

    　P18, 8.3.6 is described VEN_L, which is correct?

    ②Nominating improve idea is EN voltage change faster turn off time, is this valid idea?

    　If there any other risks, please let me know.  

    ③By the way, is VEN_L timing become output High impedance?

    Best regards,

    Satoshi

  • +1 in reply to Satoshi
    TI__Guru 57555 points

    Hello

    The output discharge FET will turn on when the VEN low threshold is reached.  It will stay on when the EN voltage reaches zero volts.

    The output MOSFETs will become HiZ and the output discharge FET will turn on when the VEN low threshold is reached. 

    The customer can try to use a faster falling edge on the EN input; but it looks to me like there is something wrong with the customer load.

    Thanks

  • 0 in reply to Frank De Stasi
    Guru 19495 points

    Hi Frank

    I feedback about additional test results,

    ・When EN off time become faster, but Vout problem is remaining.  

    　※ch1: Vin, ch2: Vout, ch3: EN

    ・When add schottky diode on Vout side for reverse blocking, Vout problem was not improved.

    From the above, is LM63635-Q1 possibility to oscillating itself?

    Best regards,

    Satoshi

  • +1 in reply to Satoshi
    TI__Guru 57555 points

    Hello

    I did not see this issue on our EVM.

    Can you try to either speed up or slow down the voltage ramp on the input, and test.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Genius 10655 points

    Hello Satoshi,

    I have not seen a reply from you in a while so I will close this thread out. 

    If you have more questions, please post a reply. 

    Regards,

    Harrison

  • 0 in reply to Harrison Overturf
    Guru 19495 points

    Hi Harrison, Frank

    Thank you for support,

    I will close and resolved this thread, these problem was cleared.

    I found the reason for unstable power off, it was overdriving from output side.

    (Uncontrolled power switch from one of output line) 

    Best regards,

    Satoshi