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TPS7H5008-SEP: Transconductance control loop

Patrick Willard
Prodigy 60 points
Part Number: TPS7H5008-SEP

I plan on using the TPS7H5008-SEP for a push pull converter and am looking into designing the control loop.  In the past I have used voltage op-amps, but noticed that most controllers leverage a transconductance amplifier for type 2 compensation.  I found this TI article partially explaining why. Article Link: Switch-mode power converter compensatin made easy.  I am refering to this line on page 17:

 "In a transconductance amplifier, there is no local feedback; therefore there is no virtual ground. You can no longer ignore the bottom resistor of the input divider, and gm can vary depending on the integrated circuit design. A transconductance amplifier is also well suited for Type II compensation."

Can you explain the no local feedback of the transconductance amplifier and how it is apparent in the voltage op amp?  I am confused how there is no virtual ground, are the inputs to a transconductance amplifier not high impedance where they cannot sink any current?  

It has been a minute since I have used an Op-Amp and can not remember the last time I saw a transconductance amplifier, so if you could break it down to a low level it would be much appreciated.

  • +1
    TI__Mastermind 24975 points

    Hey Patrick,

    The reason we use a transconductance amplifier is a bit simpler than that.
    We simply found our OTA (operation trandconductance amplifier) worked best under radiation.

    As for the comment in the sentence in the app note, its trying to explain the differences in gain equations that happen due to the different structure of the EA.
    To quote the paragraph for a traditional EA:
    "Both pins were at the same potential and the AC contribution of the lower resistor was nonexistent"
    The difference for a OTA is that the lower resistor can't be ignored for the gain equations.
    This is in part due to the different connections of the compensation.

    Quoting page 15:
    "You can analyze this circuit by recognizing that the error amplifier is inverting and has a virtual short between VFB and VREF"

    Thanks,
    Daniel