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TPS5450 HICCUP mode

mario.herran
Prodigy 50 points
Other Parts Discussed in Thread: TPS5450

Hello, I'm working with ST's ST1S14 for a 24V-in 5V-out 3A dc/dc. This part is very similar to TPS5450 in pinout, specs and functionality. The problem is the IC enters HICCUP mode at the end of the soft-start cycle with full load connected (resistive load of ~1.5ohm). This is not observed with no load. Also, under short overload transients, the IC occasionally goes to HICCUP mode. In all these cases, the IC should never enter HICCUP mode because of two facts: frist, cycle-by-cycle current limit should act before; second, we measured load current and it never exceeded cycle-by-cycle current limit. I wrote to ST's tech support with no response. I'm thinking about replacing this part with TPS5450, which has almost the same pinout, but I wonder if this part will not exhibit the same behavior.

Do you know if TPS5450 could enter hiccup mode under some cases different than the specified in the datasheet, that is a deep short-circuit that causes overcurrent runaway? I've observed the hiccup current limit is not specified in the datasheet (only the cycle-by-cycle current limit, which is between 6A and 9A). Can you provide min/max values for such current limit?

Thanks.

  • 0
    TI__Guru**** 191445 points

    The TPS5450 has both pulse by pulse and hiccup mode current limit.  The pulse by pulse is activated first and starts a counter.  If enough consecutive pulses are in current limit then teh hiccup mode is activated ( I do not remeber if it is a 2 or 3 bit counter).  Usually it is difficult to get it into hiccup mode.  When pulse by pulse current limmit occurs, the on time is decreased on the off time increases so the inductor current will ramp down further before the next pulse occurs.  So it is difficult to get successive pulses to increment the counter to hiccup mode.  So for TPS5450, the hiccup mode and pulse by pulse mode are the same threshold current.  Sometimes with any converter you may have issues with start up.  During slow start, the output voltage is rising at a constant rate.  You are also charging the output capacitance.  Since C and dV/dt are constant, a constant additional current is required to charge the output capacitance in additon to what you may be supplying the load.  For large output capacitance and fast SS times, that can cause current limit to trip.  The min typ and max current limit is in teh datasheet for TPS5450