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TS321-Q1: TS321 overshoot and unstable output

Part Number: TS321-Q1
Other Parts Discussed in Thread: TS321, , OPA192

Hi team,

My customer is using TS321-Q1 in buffer circuit for DC input voltage sensing. However, during every switching rising and falling edge of main DC/DC converter, TS321 output will have glitch and overshoot. Please see waveform and schematic below:

CH1 TS321 VCC (Pin 5)

CH2 Buffer_OUT (Pin 1) 

CH3 Buffer_IN (Pin 3)

CH4 Main DC/DC Converter Switching Node

And we changed the op-amp from TS321 to OPA192 (keeping all surrounding components the same), this problem is gone. Could you suggest what are the op-amp spec differences may lead to this improvement?

OPA192:

CH1 TS321 VCC (Pin 5)

CH2 Buffer_OUT (Pin 1) 

CH3 Buffer_IN (Pin 3)

CH4 Main DC/DC Converter Switching Node

Many thanks.

Best Regards,

Wei-Hao

  • Hi Wei-Hao,

    I think this has to do with the better PSRR (power supply rejection ratio) of OPA192.

    Take care, you should not have these heavy noise spikes superimposed to the supply voltage of an OPAmp! The supply voltage of an OPAmp must be clean, stable and noisefree. I would try to low pass filter the supply voltage of TS321. Insert a ferrite bead in the supply line, so that a low pass filter is formed. To avoid resonance you might need to put a small resistor of a few Ohms in series to the ferrite bead.

    Kai
  • Hi Wei-hao,

    Kai's answer is correct. Especially for high frequencies, the PSRR for an op-amp can be poor. The TS321 in particular doesn't have very good PSRR so switching to the OPA192 and seeing better results makes sense.

    Be sure to place decoupling caps close to the op-amp pins and use good PCB layout to help mitigate this effect.

    Thanks,
    Paul
  • Hi Wei-hao,

    I have a different opinion; I believe output current on Vbuck_diag changes causing the output current of the main class B internal driver switch from sourcing (NPN darlington) to sinking (PNP emitter follower) [see device schematic in datasheet]. Try a 1k load resistor from pin 1 to ground as a test to see if I am correct.

    The check out application note for this section :4.3 Output Crossover Time Delay