• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 961 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS7B7702-Q1: Poor Transient Response

Ken Henderson
Intellectual 670 points
Part Number: TPS7B7702-Q1

Hello,

I have designed this part into our equipment and found the transient response is very poor. Here is the circuit:

The resistor R37 has been adjusted for 7.5V output. Here is the transient response:

ChA is the output and ChB is the control signal for a 100R load switched onto the output, so the load changes from 0 to 75mA.

I see that the output drops 500mV over the first 72us after the load is turned on. This is a huge drop. Much bigger than seen in the waveforms in the ref design TIDUDK3. e.g. Fig 22 shows only about 200mV for a 120mA step:

Both my design and the ref design are using a 10uF output capacitor.

Any suggestions as to why I am seeing such a large drop on the transient response?

Thanks for any help!

Best regards, Ken

  • 0
    TI__Guru 55940 points

    Hi Ken,

    There are multiple factors that will impact the transient response of a linear regulator.  Aside from the output capacitance, factors include the magnitude and slew rate of the transient as well as the starting point of the transient.  Do you know the slew rate for your load transient?  If your load switched faster than 5 mA/us we would expect the transient to be worse.  In addition, please note that you are starting from a much lighter load from the TI Design.  When starting from a no load or extremely light load condition, the internal error amp must drive the gate voltage from rail to rail in order to respond to the transient.  Adding a small "dummy load" such as a resistor to GND may help your transient performance.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Intellectual 670 points

    Hi Ryan,

    Our load transient is very fast with rise time from 0-75mA in nanoseconds, so that will be part of the issue. Although there is also local decoupling near the load which should take care of some of the transient.

    Good call on the error amp response time: I added a 1k (7.5mA) dummy load and the transient response now looks much better! About 5x improvement.

    So we will add this to our design. Thank you!

    Best regards, Ken