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Problems with TPS40200

Kyle Aven
Intellectual 950 points
Other Parts Discussed in Thread: TPS40200

I have an issue when trying to use the TPS40200 to power a digital servo at 6V from an input voltage of 12V. The power supply works fine under resistive load conditions up to 1.65A but constantly reset from 0V to 6V when trying to power the servo. The current limit is set to 5A and the servo only uses 160mA. Is there some sort of impedance mismatch problem with using the TPS40200 to drive complex loads?

 

please help

 

thanks

Kyle

  • 0
    TI__Guru 56965 points

    Kyle,

     

    The TPS40200 controller current limit function is very fast, but if you programmed the current limit for 5A and you're only able to support resistive loads to 1.65A, it is likely that there is a time-constant mismatch between the IC's VIN pin and the ISNS pin.

    If you have selected to use a filter between the power-supply input voltage and the IC's VIN pin, it should have the same time-constant as the filter between the ISNS pin and the current sense resistor.

    It is also likely that the servo has a much lower initial input impedance and could be drawing very high peak currents.  The TPS40200's current limit can react to load current changes near the switching frequency, so even a few micro-seconds of high load current could result in triggering current limit.

    Check the time-constants on the VIN and ISNS pins and also check the input current into the servo to look for peak higher peak currents at turn-on or turn-off.

  • 0 in reply to Peter James Miller
    Intellectual 950 points

    Peter,

    Thanks for the reply. I have disabled the current limit on the TPS40200 by shorting VIN and ISNS pins. After furthering testing is seems that we can power other models of digital and analog servos, but not the one we have choosen for this design. If we place a linear regulator between the output of the TPS40200 and the servo, we can power the power the motor forward, stop, forward, stop.....But as soon as we add reverse to the equation it fails and dosen't recover. During the failure, the power supply is outputing 2.5V instead of 6V. When we look at the gate driver signal for the PFET it no long resemmbles a square wave. It is very noise and distorted. This only happens with the new digital servo so i do not believe it is a problem with the TPS40200 or our PCB layout. We believe that the digital controller in the servo is getting stuck in some unknown state. We are now trying to isolate the motor from our PCB to reject whatever noise is interfering with the gate driver signal. We have had zero luck contacting the servo manufacture to find out what changed from one year model to the next.

    Thank you for your input

    kyle