• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 63 subscribers
  • Views 1342 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.

TPS92512 PWM startup response time help needed

Steven
Prodigy 30 points
Other Parts Discussed in Thread: TPS92512, TPS92515

Hi,

I'm having trouble getting TPS92512 evm board to regulate current fast enough. After the cirquit has setteled(PWM low) for 1-2 seconds, the time for output current to reach the setpoint takes much longer compared to continuous PWM switchig.

Startup example after some 1-2seconds pause between PWM enable: blue is current (100mV = 1A), yellow is trigger (actual PWM will be high after 20uS of trigger high...)

In the needed application, the LED current should get to setpoint as fast as possible and stay there, no oscillations....

Pre biasing BOOT pin did not help too much... The application is photography, it is extremely important to have uniform pulses of light... 

When pulsing PWM with say 50mS on / 50mS off continuously, the current regulation is very nice stable, but after some inactivity, the first 'coming up' is slow.

Thanks for  feedback!

  • 0
    TI__Mastermind 25500 points
    When PWM is low the boot cap will discharge after some time and the device will enter boot uvlo. What input/output voltages are you using? Providing an external bias to boot should work (limit the current to 1mA into boot) but only if there is enough headroom between the bias voltage (usually Vin) and the output voltage which is where the PH pin generally is after the inductor current falls to zero. If that voltage differential is not greater than the boot uvlo voltage it will not work. If you need a much faster responding device you could also consider the TPS92515. It is hysteretic and has a much faster response time. It also will not enter a boot uvlo scenario as it keeps boot charged when PWM is low.
  • 0 in reply to Clinton Jensen
    Prodigy 30 points
    Thanks Clinton for the great information and recommendations.
    TPS92515 looks much better fit for our application.

    The requirements for the board are specified as:
    Vin 24V, Vout min:1.5V to max:24V
    Iout analog adjustable min:20mA to max:1.5A
    PWM used as pulse input, probably min:1ms to continuos
    Output current ramp-up time, <100us is acceptable

    What do you think could be the best possible driver chip, solution to satisfy these requirements?

    Thanks
  • 0 in reply to Steven
    TI__Mastermind 25500 points
    If you can limit the maximum output voltage to a little less than the minimum input voltage the TPS92515 would be a great fit. But if Vout needs to be as high as Vin your current would drop a little at that point. If you truly need that capability then you would need to use a buck-boost and that is a bit more complex. But if you need to let me know and I can make recommendations.
  • 0 in reply to Clinton Jensen
    Prodigy 30 points

    Usually, when 24V led string is used, the strings itself are really low current, maybe 100mA max. For me looks, that TPS92515 is the way to go.

    If there is something you would recommend to read etc. regards to TPS92515, please let me know.

    Thank you!

  • 0 in reply to Steven
    TI__Mastermind 25500 points

    It's a pretty straightforward device, no stability issues or anything, so the datasheet should be enough. There is an EVM available if you would like to try it out. There are also spice and TINA models as well as an excel calculator on the web if any of those would help.