TPS548D22: How to improve transient response and phase margin of TPS548D22

Ryan Wu Jr.

Part Number: TPS548D22

Hi e2e,

Could you please advise the way to improve phase margin and transient response? typically, phase margin and transient response are trade-off, but now we have "okay" phase margin and slow transient response.

We got 12V input, 1.88V output with maximum 25A load. The phase margin is ~46 degree at maximum load and ~50 degree at light load.

Our customer wants to have 5 more degree margin at both light and maximum load.

The following ways had been tried:

1. Changed Cff to 4700pF, 5600pF, 8200pF => the phase boost doesn't improve phase margin much.

2. Changed R_mode to 60.4k-ohm to decrease R-C ramp slew.

3. Remove two 220uF output capacitors.

over 4 years ago

0 Ryan Wu Jr. over 4 years ago

Prodigy 30 points

0 Gerold Dhanabalan - Online Design Tools over 4 years ago in reply to Ryan Wu Jr.

TI__Genius 14455 points

Hi Ryan

I am looking into this. Will get back to you by end of Thursday.

regards,

Gerold

0 Ryan Wu Jr. over 4 years ago in reply to Gerold Dhanabalan - Online Design Tools

Prodigy 30 points

Hi Gerold,

Are there any update?

0 Kris F over 4 years ago in reply to Ryan Wu Jr.

TI__Genius 17905 points

HI Ryan Wu Jr.,

Can you try removing C297 or replacing it with 10pF? The effective RC low pass filter formed by your feedback divider resistors (Ra and Rb) and the C297 1nF capacitor will have its pole in the 35kHz range, and may be lowering the phase (and phase margin) at your crossover frequency. I would also recommend reducing your feedforward capacitor to the 100-200pF range so that the maximum phase boost occurs in the 100kHz range.

If you would like to have some differential filtering in the feedback path, I recommend placing the R324/R322/C297 RC filter before the Ra and Rb resistor divider such that the resistor divider does not contribute to the RC filter time constant. Keep the RC cutoff of the filter at or above the switching frequency so that it does not pull down the phase significantly at the loop crossover frequency.

By the way, in case you use SIMPLIS, there is a model available to download on the TPS548D22 product folder.

Regards,
Kris

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TPS548D22: How to improve transient response and phase margin of TPS548D22