TPS61021A: feedforward capacitor selection

Bart

Part Number: TPS61021A

Team,

we have two questions related to the choice of feedforward capacitor (C3). In our system we have around 47pF (so far), and:

-if the effective output capacitance in the system is greater than 3000uF, what fFFZ should be chosen?

-is it possible to use much bigger C3 capacitor (for instance between a few up to 20uF) to achieve slow raise of the voltage on the output and thus limit initial current consumption?

Thanks

over 5 years ago

0 Helen chen over 5 years ago

TI__Mastermind 40080 points

Hi Bart,

Please share your Vin/Vout, Iout?

For the TPS61021A, we need to analyze case by case.

0 Bart over 5 years ago in reply to Helen chen

TI__Expert 6470 points

Hi Helen,

Vin 2,5-3,3V (lithium batt.).

Large source resistance depending on application (0.75R or 0.28R) which can affect stability.

Vout 3.8V.

Iout around 0.3A, but up to 2.2A in peaks (GSM modem, that's where 3000uF comes from).

0 Helen chen over 5 years ago in reply to Bart

TI__Mastermind 40080 points

Dear Sir,

For the feedforward capacitor, you can choose around C3=1nF-2.2nF. Too big C3 is not suggested since it will affect stability.

3000uF Cout is very big, generally it is =<1000uF for the similar application.

What is the target inrush current in this application?

0 Bart over 5 years ago in reply to Helen chen

TI__Expert 6470 points

Thank you.

Regarding inrush current, we have now above 3,5A - it disables our battery protection.

0 Helen chen over 5 years ago in reply to Bart

TI__Mastermind 40080 points

What is the inrush current in the 1000uF condition?

0 Bart over 5 years ago in reply to Helen chen

TI__Expert 6470 points

Sorry but I don't understand your quesiton. We have 3000uF, so not sure what would be our inrush current at 1000uF (i.e. that would be a different system than ours).

Could you clarify?

0 Bart over 5 years ago in reply to Bart

TI__Expert 6470 points

Helen,

can you advise? Need your help so that we can progress about the topic.

Thank you.

0 Helen chen over 5 years ago in reply to Bart

TI__Mastermind 40080 points

Hi Bart,

What I mean is 3000uF is too big. You can decrease the 3000uF cap to 1000uF cap to reduce the inrush current. And this is easy to do on the board.

0 Bart over 5 years ago in reply to Helen chen

TI__Expert 6470 points

Hi Helen,

as I mentioned earlier 3000uF is the "system" Cout due to the GSM module used, it is not a 3000uF cap that was placed on board.

Therefore, it is not possible to lower that amount.

Can you address the questions from the first post in this thread?

Thank you.

0 Helen chen over 5 years ago in reply to Bart

TI__Mastermind 40080 points

Dear Sir,

With a big cap load, the suggested added feed forward zero is around or less than crossover frequency fc. you can start from f_ffz=0.5 * fc. Did you ever test the crossover frequency fc in your system?

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TPS61021A: feedforward capacitor selection