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TPS65721: Transient behaviour of the internal LDO

Lasse Korff
Prodigy 115 points
Part Number: TPS65721
Other Parts Discussed in Thread: TPS7A05, ,

We are currently looking into you PMU TPS65721, but I need a little more info on the internal LDO before we can make our choice. More specifically I need to know how it behaves under load transients. In your LDOs – for instance tps7a05 – there are multiple plots which are describing the transient behaviour (p7-9). Do you have any similar information on TPS65721? I basically need to know if the TPS65721 will kill our IC if the load is dropped too fast.

  • 0
    TI__Genius 9380 points
    Lasse,
    This question is very specific. I am passing it to the product specialist to answer. You should hear from him later today. Thank you for using E2E.
  • 0
    TI__Genius 10885 points

    Hi Lasse,

    Figure 38 in the datasheet shows a transient responses for the LDO:
    www.ti.com/.../tps65721.pdf

    However transient response will also be affected by the value of the feed-forward capacitor recommended in section 9.2.2.1.2

    Best Regards,
    Rick S.

  • 0 in reply to Richard Solarski
    Prodigy 115 points

    Hi Rick!

    By feed-forward capacitor I guess you mean the output capacitor?

    In this case I am well aware that the output transient is much affected by this. I guess that this measurement is performed with 4.7uF X5R, which is used in your evaluation board?

    Figure 38 only gives a very small part of the story. It is also quite hard to see how large the transient actually is on that plot. Also no slew rate is given.

    What I am interested in is your worst case transient, which I guess is from 200mA to 0mA step with a high slew rate similar to Figure 15 in TPS7a05.
    Would you be able to provide this value?

  • 0 in reply to Lasse Korff
    TI__Mastermind 30280 points

    Lasse,

    A feed-forward capacitor is an external capacitor (Cff) in parallel with the high-side feedback resistor, as explained in this Application Report (SLVA466).

    Cff is typically on the order of 5-20pF. This is many orders of magnitude smaller than the bulk bypass (or de-coupling) capacitor, which is simply called Cout most of the time and can range from 1-100uF or more.

  • 0 in reply to Brian Berner
    TI__Mastermind 30280 points
    Although it is not as common to use a feed-forward capacitor on an LDO, this is what the TPS65721 recommends on page 49: "A feed-forward capacitor in parallel to the resistor from Vout to FB_LDO1 is required. Its value should be based on transient performance and will be in the range from 4.7 pF to 22 pF." This will improve the transient response of the LDO.

    Although we do not have as many plots as the other device you referenced, Figure 38. Load Transient Response LDO1 and Figure 39. Line Transient Response LDO1 show the transient response of the LDO at typical conditions.

    You can perform additional transient response tests to evaluate the LDO performance using the TPS65721EVM-516.