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MC3303: oscillation issue

Brian Wang0928
Genius 14490 points
Part Number: MC3303
Other Parts Discussed in Thread: TLV9304,

We use the OPA MC3303DR to implement the ref. voltage of the -2.5V. However, we found the oscillation waveform on the -2.5V output rail of the OPA. We replaced it to On-semi's MC3303DR2G and its around circuits are identical, the issue is gone!

Both devices have the same output impedance of the 75Ohm, why TI one has oscillation problem, but ON-SEMI's one is ok?

as we know the bigger output capacitors of the OPA would lead to the oscillation on the OPA output. In this circuitry, there are only 0.1uF + 0.1uF on the U15D. It should not cause problem, right?

This ref. voltage is an reference voltage for the current sensing signals. 

Here is sch for ref. voltage.  Please help to clarify it! Very thanks. 

 

  • 0
    Guru 140200 points

    Hi Brian,

    the circuit oscillates because of C143. Connecting a big capacitance from the output of an OPAmp to signal ground is bad design practise and easily results in instability. It's pure coincidence that the circuit oscillates with the TI's part but not with the ONSEMI's part.

    As a remedy I would add an isolation resistor or use the well known dual feedback topology.

    Kai

  • 0
    TI__Genius 15410 points

    Hi Brian,

    Kai is correct. C143 and C178 MUST be removed. Having a capacitor directly on the output of the amplifier provides no benefit. If the intention of the capacitor is to provide filtering, you must add a resistor. 

    It is possible that the On-semi device will oscillate next time. To truly test the stability of the circuit a step input needs to be applied to the input. 

    Lastly, a newer direct cross for the MC3303 is the TLV9304. However, both capacitors will still need to be removed if the TLV9304 is used.

    Thank you,

    Tim Claycomb

  • 0 in reply to kai klaas69
    Guru 140200 points

    Hi Brian,

    you could add C2 and R4:

    brian_mc3303.TSC

    Would considerably improve the phase margin.

    Kai