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TPS546D24A: Is TPS546D24A's OVP mechanism same as TPS548D22?

Albert Sun
Intellectual 310 points
Part Number: TPS546D24A
Other Parts Discussed in Thread: TPS548D22

Hi,

I want to check TPS546D24A's OVP mechanism. In TPS548D22, it turns on low-side FET until NOCP is triggered, then high-side FET is turned on for minimum on time. I think it has problem when VIN<2*VOUT because high-side minimum-on time is 60ns, minimum off time is 300ns, the high-side turn-on time cannot compensate the current drop as low-side FET has to turn-on for 300ns. For example, VIN=7V, VOUT=5.5V, when triggers OVP, low side FET current drops to  -30A threshold, then it turns on high side FET for 60ns. However, for 220nH inductor, high-side FET can only pull it to -29.6A. in next cycle low side FET has to turn on for 300ns again so negative current drops to 37.1A. If this persists the NOCL has no effect to protect lowside FET. The device will damage.

What is the mechanism of TPS546D24A instead? Will it has same problem as TPS548D22 when VIN<2*VOUT?

-A

  • 0
    TI__Mastermind 48070 points

     

    The TPS546D24A's Over Voltage Protection scheme will turn-on the low-side FET to deflux the inductor and discharge VOUT similar to how the OVP on the TPS548D22, but their Under Current Protection works differently.

    Unlike the TPS548D22, the TPS546D24A operates at a fixed switching frequency.  When Under Current is detected (peak inductor current falls below -20A) the low-side FET is turned off and the inductor current is allowed to free-wheel back into the PVIN capacitor through the high-side FET body diode for the remaining portion of the switching cycle.  While the 400ns minimum low-side FET on time is maintained, the available high-side free-wheeling time is only limited by the switching frequency.  At 550kHz, upto 1.4μs of free-wheeling time is available to deflux the inductor and prevent it from continuing to drive negative current.

    When the VOSNS pin voltage reaches 0.2V / VOUT_SCALE_LOOP, the low-side FET discharge will be released, allowing the inductor current to free-wheel and preventing a negative voltage from being forced on the output.

  • 0 in reply to Peter James Miller
    Intellectual 310 points

    Hi Peter,

    Do you mean the how long the high side FET can turn on when NOCP is triggered, depends on switching frequency clock? Does this mean that even at NOCP the output current is still controlled by your VSHARE?

    -A

  • +1 in reply to Albert Sun
    TI__Mastermind 48070 points
    Albert Sun said:
    Do you mean the how long the high side FET can turn on when NOCP is triggered, depends on switching frequency clock?

    No,

    The high-side FET is not turned on, but how long the low-side FET can be turned off depends on the programmable switching frequency, and is upto Switching Period - 400ns.

    Albert Sun said:
    Does this mean that even at NOCP the output current is still controlled by your VSHARE?

    No.

    If Under Current Protection is triggered during normal operation, VSHARE will continue to decrease as the loop attempts to force even more negative current, but limited low-side FET on-time will prevent the further decrease of the inductor current.