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TPS54229: TPS54229 soft start time

Jamie Tseng
Intellectual 2405 points
Part Number: TPS54229

Hi Team,

Could you give some advice on adjusting power up time to make red behind blue?

 We have try soft start time (pin 8) but it didn't work.

Thanks and best regards,

Jamie

  • 0
    TI__Guru 52805 points

     

    If I am reading your oscilloscope waveforms correctly, the Red line is the 3.3V output from the TPS54229 and the Blue line is the 12V input to the TPS54229.

    Since the TPS54229 is a BUCK converter, its output will always be lower than its input, so the RED line will always be after the BLUE line.  They appear to cross in the attached waveforms because the Blue line uses a scaling of 2V/division and the Red uses a scaling of 1V/division.  With the current configuration, the 3.3V output reaches 3.3V before the 12V input reaches 6.6V.

    However, if you want to delay the 3.3V further in time than you currently have, there are some options:

    1) Does the converter providing the 12V supply have a power-good output that could be used to drive the TPS54229's EN pin instead of pulling the enable pin up to the 12V input with R215?  Connecting the power good output of one converter to the enable input of another converter is a common way to create sequential start-up and conditional power-down where a converter is not allowed to turn-on (disabled) until the prior supply in the start-up sequence has powered up, and will automatically shut-down if the prior supply in the sequence shuts down for any reason.

    2) You could delay the start-up of the TPS54229 by adding a capacitor from EN to GND to create a time-constant in the charging of EN so that it lags behind the 12V input start-up.  I would recommend some additional changes to make this delay circuit more effective:

    1) Change R215 from 10k to 200k.

    2) Add an additional 51k resistor from EN to GND so that the voltage at the EN pin is 1/5 of VIN instead of the full VIN voltage.

    With the EN voltage threshold of 0.6 to 1.6V, this could provide a start-up voltage range from 3V to 8V at VIN, but the lower voltages on the EN pin will make the time-constant more predictable

    3) The total resistance at the EN pin will be 37kOhms, so a 47nF capacitor from EN to GND would add a 2ms time-constant to add some additional delay between the 12V start-up and the 3.3V start-up.