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TPS71530, LM317 I/O Capacitors

Aamir Ali
Expert 2525 points
Other Parts Discussed in Thread: LM317

Hi ALL,

In one of my application I need to generate 3.0V o/p. I found two regulators LM317 (which we used earlier) & TPS71530 (which I want to use). I have several questions regarding this:

1. In datasheet of LM317, o/p cap of 1uF electrolytic is advisable & in TPS71530 its .47uF (or greater) ceramic or tantulum. Do different type of capacitors effects differently in voltage regulators. Like using 1uF electrolytic in TPS71530 produce different results as compared to 1uF ceramic?

2. In LM317, 1uF is preferred at o/p. Do that 1uF is suitable for entire current range of o/p current 0-1.5A, (limit of LM317 )or higher value cap is needed, if current increases.In brigde rectifiers I  read that with increase in current cap value also increases. Do the same applys here also.

3. What are the factors by which input & output capacitors values are calculated in an application?

4. I found some application notes, but they consider frequency factor also. Is it noise. But noise in circuit is not having some constant frequency.

Aamir Ali, TE

DAS

  • 0
    TI__Guru**** 164606 points

    Aamir,

    The LM317 is stable with any output capacitor including no capacitor over the whole current range.
    A input capacitor helps stablility. The only weakness in the LM317 is the drop out voltage is about 2V.

    The output capacitor helps with load transients. Lower ESR is better.

    Reagrds,
    Ron Michallick

  • 0 in reply to Ron Michallick
    Expert 2525 points

    Hello Ron,

     Is there any relation or mathematical formula for calculating I/O caps for optimal performance in regulators.

    Regards

    Aamir Ali

    DAS

  • 0 in reply to Aamir Ali
    Prodigy 70 points

    For switching regulators, yes, there is a specific method to calculate the correct amount of output capacitance. That is usually a function of the output L (assuming buck topology), output C, output current and desired output ripple. The controller datasheet will typically show how this is done.

    For linear regulators, you simply need to put down sufficient output cap for your regulator to be stable. Some like the LM317 do not require any, most others specifiy a minimum value. Beware that some not just require a minimum C, they require a min ESR too. Some are less picky - these are usually listed as "stable with ceramic caps". Once you meet the minimum Cout, then the next thing you do is size up the Cout as required for your load. Beware that not all regulators are stable driving extreme load capacitance.

    Take note too when selecting caps. If the datasheet says Cout min = 4.7uF, do realize that ceramic caps have reduced capacitance as their DC bias voltage goes up. This effect is more pronounced for small package sizes. So prudence dictates selecting a larger rated capacitance.

    Back to your question about optimal performance, note that bypassing the ADJ or NR pins in most LDO's help reduce the output noise (besides just worrying about Cout). Refer to datasheet/app notes for details.