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TPS56121; soft-start

Other Parts Discussed in Thread: TPS56121

Hello,

there is an issue regarding the soft-start of TPS56121.

Requirement: 3.3V with soft-start time < 400µs

=> 22nF soft-start-C like in datasheet is not applicable; I have to use 4.7nF or 5.6nF (regarding all tolerances C_SS, V_FB, I_SS)

Here you see the measured waveforms:

  • CH1 is the VCC3.3 voltage
  • CH4 is the voltage at the Enable-pin
  • this Enable-pin is controlled by an open-drain, no pull-down output of a PLD (Lattice MACH-XO2)
  • C1801 is the soft-start-capacitor

My concern is the peak in voltage at the beginning of the calibration phase. I did not see this peak when soft-start-C was 22nF!

=>

- does this peak cause any trouble?

- how high may this peak be (here it's about 2.4V;  OK < abs max ratings @ EN/SS = 7V  BUT > 1.3V = soft-start voltage level ; start of ramp)?

Many thanks

regards

Udo