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TPS81256 - minimum output capacitor

Guru 20090 points
Other Parts Discussed in Thread: TPS81256, TPS53355

Hello,

In the datasheet of TPS81256, output capacitance range is 2uF to 30uF.

If we use this device with 10mA(max) output , can we remove output capacitance ?
If no, please let me know minimum capacitance for 10mA(max) output ? 
Since there is 2.2uF*2pcs on the module, I think that we can remove external Cout for low output current condition.

Best Regards,
Ryuji Asaka

  • The D/S does not distinguish between PFM and PWM modes for the Cout requirement, so I can only recommend that you add the 2 uF Cout. Any capacitance at the input of the load counts towards this value.
  • Hello Chris san,

    Thank you for the reply. I understood that we only recommend that 2uF Cout.
    Thus, could you please let me know the following questions ?
    Since the customer would like to discharge output capacitance more earlier , they consider to use less than 2uF Cout.

    (1)
    could you please let me know the potential problem when we use 1uF for Output cap with 500uA output current ?

    (2)
    Could you please let me know the technical background of output capacitance range ?

    Best Regards,
    Ryuji Asaka
  • Ah, this is interesting. So you want to use less Cout to have a faster discharge? I assumed it was for size reasons.

    There is no discharge inside the TPS81256, so may I ask how they intend to accomplish it? Also, what is the requirement for discharge time?

    Clearly for any boost converter, the output capacitor is a critical part of the control loop for stability.
  • Hello Chris san,

    The customer add the resistor as parallel with Cout for increasing discharge time.
    The requirement time is 1us.

    Best Regards,
    Ryuji Asaka

  • Hmm, 1 usec is quite short. It also does not make sense. A ~1 Ohm resistor is required to discharge this. Is there a typo? How do they plan to discharge it? Why do they need to? To what voltage level does it need to discharge?
  • Sorry. This is typo... time is 1ms.
    Actually, the output current is 1mA. also the current discharge time is about 2~4ms.
    They would like to reduce this time as possible. ( also reduce the board area)

    They use this device for TPS53355 VDD and there is the 4.7uF for VDD pin.
    Thus total capacitance between Vout of TPS81256 and VDD of TPS53355 is 2.2uF * 2pcs + 2uF + 4.7uF = 11.1uF.
    I'm checking about VDD of TPS53355 on the Non-Isolated DCDC Forum.

    Best Regards,
    Ryuji Asaka
  • Hmm, I'm not sure that saving an 0402 or 0603 capacitor is a significant size savings when you have that 30A converter in the same design.

    Yes, you should use the 4.7uF for the VDD. This would likely meet our need for extra Cout.

    It's still not clear why you are concerned about the discharge time for this application. Is adding a fixed load resistance a possibility to speed it up?