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TPS7A88: Maximum load capacitance can support

Jonathan Geronga
Expert 7930 points
Part Number: TPS7A88

Hi Team,

We have a customer inquiry regarding the maximum load capacitance TPS7A88 Regulator can support.

The datasheet only shows a 10uF minimum output capacitor for TPS7A88.
However, what is the Maximum load capacitance the TPS7A88 support and still regulate properly?
The input voltage is 3.3VDC and the TPS7A88 output is configured for 1.2V output with a 0.182A load.

They would like to increase the load, but it would have a significant capacitance.

They use the datasheets recommended 10uF at the output and are concerned about loading capacitance causing a start-up issue or instability.

Let me know if you need more information form the customer.

Thanks!

Jonathan

  • 0
    TI__Mastermind 34225 points

    Hi Jonathan,

    The device will be stable above 10uF of load capacitance.
    Very large capacitors will often have large ESR values.
    If the load capacitor has large ESR, you will want to supplement with a parallel small ceramic capacitor to lower the effective ESR.
    Ceramic capacitors typically have ESR values less than 10 miliohms, and often less than 5 miliohms.
    I would worry less about instability.
    A more immediate concern is inrush due to startup, or a very long startup time.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Expert 7930 points

    Hi Stephen,

    Thank you for your support in this thread.
    I already forward this to our customer have followed up questions:

    "Page 8, Figure 18 shows the part will go in to current limit and foldback (like any LDO) if start-up current is an issue.
    Also, see your page 9, figure 22, soft-start will limit the initial inrush.
    We have a 12ms soft-start time set"

    The customer still concerned about loading capacitance causing a start-up issue or instability due to excessive capacitance. They also have the soft-start time set to 12ms as mentioned above.

    Please let me know if you need more information with the customer.

    Regards,

    Jonathan



  • 0 in reply to Jonathan Geronga
    TI__Mastermind 34225 points

    Hi Jonathan,

    The load capacitance should not impact the startup unless there is an inrush limitation, or the source to the LDO is insufficient in handling the inrush.

    Immediately after the input to the LDO exceeds the internal UVLO and enable trip points, the LDO will turn on according to the soft start of the device.
    It will pull current from the line to charge up the load.  In any linear regulator (not just this one), this can in some cases result in a surge of current from the input capacitor.  This can cause the input voltage to droop as charge is taken from Cin to charge up Cout but before the source power supply can react.  There is a risk that the input will droop below the UVLO trip point, or the enable point.  This will cause the output to turn on, then off, then on, and so on, until it fully ramps to steady state.  This can be avoided if the input capacitor is sized about the same or larger than the output capacitor, or by slowing down the startup.  I recommend 10x the capacitance on the input, as what is on the output, to avoid this condition, although that is not always possible when the output capacitance is very large.  The next best option is to reduce the turn on time to slow down the ramp rate.

    The linear regulator was designed to be stable with 10uF of load capacitance or more.  Does the customer have data showing an oscillation with this device with a larger amount of load capacitance?  If so, can we see that data?  You can send me a private message if the customer cannot post these details on this forum.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Expert 7930 points

    Hi Stephen,

    Customer already agreed in the above answer you shared.

    I will now put this to resolved and thanks for the informative support.

    Regards,

    Jonathan