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LMH6654: Suitable for Load Transient Circuit?

Carmen Parisi
Expert 4060 points
Part Number: LMH6654
Other Parts Discussed in Thread: LMH6723

Hello,

I'm currently using the LMH6654 in a prototype Load Transient Circuit for testing high power regulators. The amplifier drives a discrete AB output stage which then switches some load FETs on or off. The current setting pulse is provided by a function generator and a 25mV input corresponds to a 25A load step at the output of the regulator under test. GaN transistors were chosen for the load FETs to keep input capacitance down and provide faster switching.

Currently, I can get a positive slew rate of ~140A/µs for a 50A step (50mV input pulse) but I'd like to push that higher if possible. Is the LMH6654 the best choice for this application? I picked it for the high GBW and decent output current drive but if there's a better amp our there I'd be happy to use it instead.

Thanks for the help!

Carmen 

Schematics attached. 

GaN Load Gen Circuit - 20180222.pdf

  • 0
    TI__Expert 7875 points
    Hi,

    You can certainly step up to an amplifier with more bandwidth and slew rate for better drive capabilities. You could look at something like the LMH6723, which is a current feedback amplifier that would give you more slew rate and bandwidth than your current circuit. We have a lot of options to move to a faster amplifier, so it all depends on how much faster you would like to go.

    Regards,
  • 0 in reply to Jacob Freet
    TI__Expert 4060 points
    Hey Jacob,

    Thanks for the suggestion. I'll look into the LMH6723. For transient circuits there's no such thing as too fast since it's much easier to slow things down usually. Would a current feedback amp be okay in this topology since the amp is essentially an integrator? I recall from past projects CFB amps don't like pure capacitive feedback but I'm not an expert.

    Cheers,

    Carmen
  • 0 in reply to Carmen Parisi
    TI__Genius 15975 points
    Hi Carmen,

    It might be ok to use a current feedback amplifier like the LMH6723 in the topology you are using, since the feedback cap is in series with the feedback resistor.

    The capacitive feedback is usually a problem for CFB amplifiers when there is a direct connection from output to input using a cap. Adding a feedback cap in parallel across the negative feedback loop usually results in its impedance reducing at high frequencies, which is not good for the stability of the CFB amplifier.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    TI__Expert 4060 points
    Thanks Rohit,

    I've got samples of the CFB amp on the way and will give them a shot when they get here. Appreciate the help!

    Carmen