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LM27403: Design not switching on Startup

Michael McLoughlin
Prodigy 10 points
Part Number: LM27403

Tool/software:

We have a design where the design never switches on startup. A 33nF capacitor is connected to SS but as can be seen below, SS never starts to rise as would be expected. VDD is present as expected. The only difference to the schematics shown below is that the UVLO/EN is driven by the PG of another regulator. Must it be driven by VIN or can VIN be present prior to UVLO/EN? Any suggestion as to what might cause this behaviour. We have tried connecting OTP to GND and we also connected D+ to D- to GND but neither of these changes made any difference to the behavior shown below.

CH1: COMP  CH2: SS  CH3:UVLO/EN

  • 0
    TI__Mastermind 23945 points

    I will review the schematic and data, do you have a pcb layout image. 

  • 0 in reply to David_Daniels
    TI__Mastermind 23945 points

    The schematic looks like a webench schematic with an input filter added.  The datasheet states to ground sync pin when not used, I do not think this is the issue. 

    The SS pin voltage is ramping very fast, the 27nF soft start capacitor should a ramp to 0.6V in 5ms but in scope plot is ~400us.   

    Check the connection of the SS pin to the SS capacitor.   

    If the SS ramps quickly, the inrush current can be large causing the input voltage to droop or overcurrent to trigger. 

    The EN pin has an internal pullup, an open drain power good from another device should suffice.  The voltage level on the en pin is greater than the enable threshold.  The soft start ramping which indicates the enable threshold was exceeded.   

    Do you have a sw node ploted with ss and comp pin?

  • 0 in reply to David_Daniels
    Prodigy 10 points

    Hi,

    We found an issue on our schematics that was the cause of the issue and resolved our issue.