• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 441 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Global input capacitors and the TPS70950

Dave Villareale
Prodigy 20 points
Other Parts Discussed in Thread: TPS709

Hi,

I am currently building a micro controller circuit that uses a TPS70950 to supply the 5V that the processor needs.  I read the spec sheet but still  have questions concerning the input capacitor. The specs state that no input capacitor is needed but suggests using a 0.1 to 2uF capacitor installed right next to the regulator.

The circuit I am building uses a power brick that is plugged into the wall.  I usually use a diode, for polarity protection, and, say, a 470uF capacitor and sometimes a 0.1uF monolithic, in parallel, installed right next to the input power connector on the board to filter out any noise or ripple. From there, I plan on tapping off some of the input voltage to supply the TPS70950 and micro controller.

My question is, how do I coordinate the global input capacitors with the input capacitance requirements of the TPS70950?  Are the 2 sets of capacitors separate and distinct, or do they need to be separated by, say, an additional diode?  I don't know how to address this issue.

If anyone has experience with this I would appreciate some guidance.

Thanks,
Dave

  • 0
    TI__Guru 55940 points

    Hi Dave,

    The purpose of a local input capacitor is to help lower contract any line inductance upstream as well as to serve as a pre-filter for the LDO.  As you state, this capacitor is not required for the stability of TPS709.

    The other capacitors that you have on the TPS709 Vin rail do not need to be considered separate and distinct form the recommended input capacitor local to the IN pin.  You do not need to place anything between the capacitor sets and the closer you place the "global input capacitors" to the IN pin of the LDO, the more the these capacitors simply become one capacitor bank.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 20 points

    Ryan,

    I re-read my post and realized that I may not have been clear in my statement.

    The circuit that I am building is an audio circuit and I an using the large value cap to clean up the power coming into the board. From that power, I am planning on taping off some of it to power the tps70950 and the micro controller and its circuits.

    I just wanted to make sure I understand correctly . My take away from what you said that the extra capacitance that I am adding on the input side of the TPS70950 is not an issue and it would even be better if I place these extra capacitors close to the TPS70950.

    Correct?

    Dave

  • 0 in reply to Dave Villareale
    TI__Guru 55940 points
    Hi Dave,

    Correct, the additional capacitance on the input supply will not negatively impact the TPS709 performance. We generally recommend an input capacitor close to the IN pin in order to counteract reactive input sources.

    For TPS709 we are far more concerned with the output capacitor location than the input capacitor location. Cout should be placed as close to the OUT pin as possible with a direct, same layer connection to the GND pin. It is the output capacitor that is compensating our internal control loop.

    Very Respectfully,
    Ryan