This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS51218: device enable with RF resistor connecting to external power supply

Part Number: TPS51218

Hello,

Our customer has a question if TPS51218 enables with connecting the RF resistor to a external power supply.

The datasheet specifies RF resistor should be connected to PGOOD in FCCM operation though the customer would like to do it to an external power supply. Would you please let us know if TPS51218 works normally in FCCM with RF resistor to external power supply such as V5IN.

When I checked on the datasheet, it specified that applying 0.1V to RF in order to measure a switching frequency at a beginning of enable state. So I think that TPS51218 banned such application.

Could you please let us know if the customer can use TPS51218 with connecting the RF resistor to a external power supply.

Best regards,

  •  

    The TPS51218 requires a low-impedance path to the local GND of the TPS51218 at power-up to support external resistor detection.  Connecting the RF pin to a external supply that does not provide a low-impedance path to ground during the TPS51218's power-up will present a problem for oscillator programming.  \

    While it can be pulled up to an external supply after initial power-up, it should not be directly connected to an external supply initially.  That is why PGOOD is recommended for FCCM programming, at power up PGOOD provides a low-impedance path to GND, allowing RF detection.  During Soft-Start, PGOOD remains low, holding the IC in auto-skip mode during soft-start to prevent it from sinking current from a pre-biased output voltage through diode emulation and DCM operation.

    If the customer is looking to enter FCCM operation before PGOOD is released, they will need to build their own MOSFET pull-down to provide a low-impedance path to GND, then release that pull-down prior to PGOOD being released.

  • Thank you very much for your detail explanation why RF resistor has to tie to GND when it starts up.

    Best regards,