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Original question:

TPS7A49: Should we use VFB or VREF of TPS7A4901 to calculate Vout

TPS7A49: LDO turned OFF during switching.

yathavakannan viswa
Prodigy 70 points
Part Number: TPS7A49

Tool/software:

We are using this LDO (15V to 6.5V) in the bootstrap circuit to power the high-side FET in a PSFB ZVT converter. During module switching, we have observed that the LDO intermittently turns off. Please refer to the attached schematic and waveform for more details. Could you help us understand why this occurs and how we might resolve it?

CH-1 Switch node.

CH-2: Boot LDO output.

  • 0
    TI__Mastermind 37065 points

    Hello,

    Please check the first PCB layer directly underneath the LDO.  Is it all GND?  Or is there copper attached to another signal (such as the switch node) directly underneath the LDO and around the FB pin?  If you can snapshot the layout it would help to confirm this is not a capacitive coupling between plane layers.  The FB pin is unfiltered and the most sensitive node in an LDO (not just this LDO but any LDO), and switching noise on an adjacent layer can cause problems like this to occur.

    Other troubleshooting suggestions:
    1. Can you provide differential measurements (with respect to GND of the floating LDO) of Vin and Vout (so we can see how Vout changes with respect to Vin)?

    2. Can you try reducing the series 3.3 ohms, or increasing the shunt Vin capacitance (or both)?  I'm concerned that Vin is drooping and possibly shutting down the LDO by tripping Ven threshold or even a UVLO threshold.

    Thanks,

    Stephen