TPS62400 cap to adjust output

Aditi Akula

Expert 1440 points

Other Parts Discussed in Thread: TPS62400

Hi,

I am using the TPS62400 chip in my module and have a quick question.

On page 28 of the datasheet (SLVS681D), a 33pF cap Cff2 is placed across R21 (looks like

adjusting cap) for one voltage output.

But there is no such adjusting cap for the second voltage output (across R11).

Can we add a 33pF cap across R11 too or is there a specific reason not to add the cap there ?

Thanks,

Regards,

Aditi.

over 14 years ago

0 JohnTucker over 14 years ago

TI__Guru**** 191445 points

That capacitor is a feed forward capacitor. It is used to boost the phase margin. The amount of available phase boost is dependent on the output voltage. For lower output voltages, the amount of phase boost is low, so teh feed forward capacitor is not effective. The circuit of fig. 41 only uses the FF cap for the 2.5 V output not the 1.5 V output.

0 Matthew Day over 14 years ago

TI__Mastermind 36110 points

As John mentioned, because output 2 is a lower voltage, adding a Cff capacitor does not provide much benefit. If you graph the transfer function of the feedback network on a semi log scale (Bode Plot), it will easy to see that the lower the output voltage, the lesss phase boost provided by a Cff capacitor. You can add the Cff capacitor if you want to. The Cff capacitor will not cause any reduction in performance.

0 Aditi Akula over 14 years ago in reply to Matthew Day

Expert 1440 points

Michael,

I am using the dual-out switcher to get two voltages 1.6V and 1.2V.

Since both are low voltages, can I remove the Cff cap on both or add the cap on both?

Please confirm it will not effect the performance.

Thanks,

Aditi.

0 Matthew Day over 14 years ago in reply to Aditi Akula

TI__Mastermind 36110 points

Yes, you can add a Cff capacitor to both outputs without negatively affecting performance.

0 JohnTucker over 14 years ago in reply to Matthew Day

TI__Guru**** 191445 points

Just FYI, here is a table that shows the maximum theoretical phase boost as a function of output voltage for a feed forward capacitor.8765.FF_phaseboost.pdf

As you can see the amount of useable boost is low for lower voltages

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TPS62400 cap to adjust output