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TPS61256A: How TPS61256x implement output shorted/over current protection

Fandy
Expert 2285 points
Part Number: TPS61256A


Dears

How TPS61256x implement output short/over current protection?

  • 0
    TI__Mastermind 40080 points
    When the load current increase , and makes the valley current touches the current limit value, the output volatge will drop. When the output current further increase, and makes the volatge further drop to a very low value, both the high side FET and the low side FET off.
  • 0 in reply to Helen chen
    Expert 2285 points

    Hi Helen

    Could you explain it in more detail ? thanks

    Datasheet ：“ When the load current is increased such that the lower peak is above the current limit threshold, the off-time is increased to allow the current to decrease to this threshold before the next on-time begins (so called frequency fold-back mechanism). When the current limit is reached the output voltage decreases during further load increase ”

    It seems IC no limited the current, just detect the valley current, if valley current exceeds the Current Limit Threshold(Typical 2.4A) ,then extend the off-time but on-time is constant to limit the Average current ,but Ipeak dosen't limited ? Ipeak  is defined by the on-time and the inductance,based on the VIN/VOUT ratio, a simple circuit predicts the required on-time.so how to calculated ON-TIME ? 

  • 0 in reply to Fandy
    TI__Mastermind 40080 points

    Peak current is limited by the Vin, inductance, and Ton. Ton is determined by the Vin and Vo(the IC sample the Vin, Vo value and calculates it). When the IC enters into the current limit condition, Vo begin drop, then Ton will also drop. So you need't worried about the Ipeak.

  • 0 in reply to Helen chen
    Expert 2285 points

    Hi Helen

    1、So you mean the Ton is  inversely proportional to Vout ? Could you give the Ton 's Equation?

    2、In my understanding: when in Current Limit Operation, Ipeak=I valley + Vin*Ton/L,(I valley=2.4A typ) , is right? so how to calculate Ton?

  • 0 in reply to Fandy
    TI__Mastermind 40080 points
    Yes, Ton is inversely proportional to Vout. it calculates by Ton=Ts *(1-Vin/Vo)
  • 0 in reply to Helen chen
    Expert 2285 points

    Hi Helen

    According to Ton=Ts*(1-Vin/Vo)  ; Ton is direct proportional to Vout , not inversely proportion 

  • 0 in reply to Fandy
    TI__Mastermind 40080 points

    Yes, you're right. When Vout is lower, Ton is lower.