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TPS92520-Q1: SC waveform comfirmation

zhiyuan zhang
Intellectual 620 points
Part Number: TPS92520-Q1

Hello expert,

    Here is my test waveform. The left is before the SC event, and the right is about after the SC event.Pls check my question:

    1.FLT keep on low before and after the OC.I know KLT low represent a fault detected, so why low before OC happened?

    2.When OC happens,SW voltage become higher, does these due to the oscillation of the harness and the output cap?What is the risk?

    3.What is the frequency of SW after OC depends on?

    4.output current keep the same,so the devise will remain the output current when OC,unless the MCU detect and do the strategy?(such as disable the channel) as is said in chapter 7.3.14

   

  • 0
    TI__Mastermind 18965 points

    Hello Zhiyuan,

    1.)  You need to do a write to clear or reset the FLT pin.  This is covered in the datasheet.

    2.)  I am assuming OC is over current, or maybe we should say short circuit (SC).  This is a CC (Constant Current) buck which means it maintains the current even into a short.  I think that change in visible waveform might be an artifact of your how your scope samples the data.  You need to zoom into the waveforms to the time where the HS FET is on. I also mean to zoom in on time, not the zoom function of the scope.  Make sure the samplings is at its highest resolution too. 

    3) I am not convinced the SW node voltage is higher, as I said before about the scope zoom and settings.  You might see that the switching frequency changes because you might be hitting the min on time of the device.  This happens when Vout is small (like a short) and Vin is high (like at high voltages such as 60V) and when the switching frequency is high (this sets the switching period).  The control loop will try and maintain the duty cycle to maintain the current setpoint.  If Vout/Vin (which is duty cycle) * TON (setpoint for switching frequency in registers) is larger then the min Ton then the switching frequency is the same as it is set for normal operation.  If the on-time is less than min Ton then the off time is increased until the duty cycle is maintained.  That means the switching frequency will foldback (go lower) to maintain the duty cycle that controls the current into the load (in this case a short).  Keep in mind that the losses affect the duty cycle because some of that on time has to go into offsetting the losses.  But in general the switching frequency will depend on the short voltage (Resistance of short * CC setpoint), Vin, TON setpoint (sets period or switching frequency), and the min on time (which is typically 105 ns). 

    4)  This is up to the system level requirements, you can continue to switch into the short or determine that it is in that condition and do something else.  You need to assess that for yourself and make an engineering judgement based on your system level requirements.  

    -Francis Houde