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Using the TPS40170 as a current source

Malcolm McClure
Prodigy 140 points
Other Parts Discussed in Thread: LMV321, INA198, TPS40170

I am trying to use the TSP40170 as an adjustable current source.

If I wrap standard voltage feedback around the circuit, everything works fine.  I get a regulated voltage at up to 10 amps.

If I use a high-side current sense (INA198 looking at a .003 ohm resistor) and an op-amp (LMV321) to provide the feedback, I get nothing.  The FB pin measures .043V so I would expect the switcher to turn on to its maximum amount.  Instead I just see an occasional narrow spike on the HDRV line and get zero current out.

Any idea what's going on?

  • 0
    TI__Mastermind 35165 points

    Hi Malcolm,

    Could you share the schematic of the circuit you tested with the INA198 and LMV321?

    What may be happening is that with the constant current circuit, the TPS40170 is trying to source too much current causing the hiccup current limit to be triggered. The easiest way to confirm this is to measure the waveform on the SS pin. If current limit is being triggered you would see a sawtooth waveform.

    If you find that hiccup current limit is the issue, you may need to add some type of soft-start circuit for the LMV321 so it doesn't try to demand 10 A right away. Alternatively this may also be a compensation issue with the constant current circuit.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 140 points
    Anthony,

    I'd be happy to send you a schematic as long as it is kept propriatary. How do I attach it?

    Your idea that I am tripping the current limiter is one I have considered myself. It is difficult to examine this, however, because of another issue that I has been plaguing this design. Despite all the protections that are supposedly built into the TPS40170, the chip seems remarkably susceptible to damage. I have been through 10 parts already and the symptoms are always the same. The chip gets hot and the and the internal supplies (8V and 3.3V) sag. This is an un-recoverable fault since I can disconnect the FETs and the chip still gets hot though it is no longer driving anything. The FETs are un-damaged in this process. I can replace the chip and reconfigure the circuit as a voltage source. Everything works.
  • 0 in reply to Malcolm McClure
    TI__Mastermind 35165 points

    To keep it private, you could send it directly to me only by starting a conversation. An option to do this should pop up if you mouseover my name above my post. Also if you can, please send the layout.