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TPS548A28: The chip cannot realize overcurrent protection

yangshuping
Prodigy 10 points
Part Number: TPS548A28

The overcurrent protection current we designed is 16.8A, but the chip is still unprotected when the load current reaches 18A. After that, I set rtrip to 5.6k on EVB (Replace the chip on EVB with tps548a28), that is, the overcurrent point is about 12A. At this time, the chip limits the current to 13A, but still does not protect the chip. What is the reason?

The current is limited to 12A

Figure 1 The current is limited to 13A                                           Figure 2 Schematic diagram of EVB：R1=5k.R2=20k,RCS=5.6k

  • 0
    TI__Mastermind 32030 points

    Hello ,

    the reason for that is because the output overcurrent limit (OCL) in the device is implemented using a cycle-by-cycle valley current detect control circuit. The inductor current is monitored during the on-time of the low-side FET. 

    So the actual average current limit will be affected by the inductor ripple and will be higher that the valley. see this post, it explains the Valley 

    https://e2e.ti.com/blogs_/b/powerhouse/posts/understanding-valley-current-limit

    Can you please provide a plot with Inductor current and measure where the valley is, it should be within the min max of the datasheet spec. 

    Thanks! 

    Tahar

  • 0 in reply to Tahar Allag
    Prodigy 10 points

    Do you mean that the measure taken by the chip for overcurrent is to limit the current rather than turn off the output？What kind of situation does the picture in the datasheet indicate?

  • 0 in reply to yangshuping
    TI__Mastermind 22440 points

    Yang Shuping,

    The device implements a cycle by cycle current limit until the output voltage drops 20% and the device restarts.

    The current limit calculation is the max current that the device can output and still maintain output voltage regulation. 

    Your oscilloscope plot shows the output voltage drooping to 680mV, the device is current limiting. 

    If the output current continues increasing, the output voltage will droop until the hiccup and restart. 

    The image shows the OCP and the 68us wait time for UVP triggering. 

    Regards,

    David