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TPS7A87: TPS7A87 PSRR measurement result is much lower than that of the datasheet in high frequencies

Xiaopeng Wen
Prodigy 90 points
Part Number: TPS7A87

Dear team,

   I measured PSRR of TPS7A87 based on TPS7A87EVM these days, and I found that the result is close to the value of datasheet in 10Hz-100KHz, but the peak value is 20dB in 1MHz-10MHz, which is much lower than that of datasheet.

I read some LDO PSRR documents, and I think that output capacitor may decrease the PSRR in high frequencies. The ceramic output capacitor on EVM is 10uF (0603). I want to know why my measurements are much lower than the datasheet in high frequencies?

Attachment

1. TPS7A87 datasheet

2. TPS7A87EVM User's Guide

3. output cap(C4) in tps7a87evm

4. measurement result1 vout=4.2V_vin=4.3V_I=0.02A_rip=100mV

5. measurement result2 vout=4.2V_vin=4.5V_I=0.02A_rip=200mV

6. LDO PSRR document

http://www.ti.com/lit/an/slyt202/slyt202.pdf

http://www.ti.com/lit/an/slaa414a/slaa414a.pdf

7.My method of measuring PSRR

  • 0
    TI__Guru 55940 points
    Hi Xiaopeng,

    You are correct that for frequencies beyond 1 MHz the LDO's contribution to PSRR is negligible and the PSRR is dominated by the output capacitor.

    In your setup, did you remove the input capacitor on the TPS7A87 EVM?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Prodigy 90 points

    Hi Ryan,

    Thank you for your reply.I didn't remove the input capacitor on the EVM. Attachement is my measurement method diagram.Noted that I added a 330uF and a 8 Ohm resistor in input pin.

    Regards,

    Xiaopeng Wen

    AFAA intern in Beijing site

    E-mail: xiaopeng-wen@ti.com

  • 0 in reply to Xiaopeng Wen
    TI__Guru 55940 points

    Hi Xiaopeng,

    We remove the input capacitor whenever possible when we take PSRR measurements as the input capacitor will decrease injected ripple.  

    Some regulators require an input capacitor for stability.  For these regulators we use the smallest tantalum capacitor possible in order to reduce the attenuation of the input signal.

    Very Respectfully,
    Ryan