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Original question:

TPS5450 HICCUP mode

TPS5450: TPS5450 Hiccups issue

Yang Wu
Intellectual 2690 points
Part Number: TPS5450

Hi Team,

it seems I met a similar issue as https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/216974/tps5450-hiccup-mode 

Issues and behaviors are (Note: The TPS5450 is designed for 18V nominal voltage, 30V max, 15V/2.33A output):

1) short circuit test @18V input, 15V/2.33A full load, TPS5450 can restart successfully when short circuit condition is removed (still with full load).

2) short circuit test @30V input, 15V/2.33A full load, TPS5450 cannot restart successfully (behaves still hiccups) even short circuit condition is removed (still with full load). it can only restart after powerup again (still with full load). 

3) short circuit test @30V input, 15V/2.33A full load, TPS5450 can restart successfully when circuit condition is removed and load changed to 1.5A.

4) Power up @30V input, 15V/2.33A full load, it can start normally.

In https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/216974/tps5450-hiccup-mode, it says "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 addition to what you may be supplying the load.  For large output capacitance and fast SS times, that can cause current limit to trip."

In my case, when it's full load, the charging current is larger and may exceed the threshold to trip. my question is, Is the "slow-start" described in TPS5450's datasheet the same as "soft-start" that we usually see in other TI products? normally, "soft-start" refers to Vref gradually increase during powerup. Here, i don't know how the "slow-start" works. Could you help explain? In addition, if "slow-start" equals to "soft-start", why case 2) can't work but case 4) can work?

thanks and looks forward your reply.

Best regard,

YANG

  • +1
    TI__Mastermind 41760 points

    Hi Yang,

    Can you help me understand what the difference between Case 2 and Case 3? It looks like both are short circuit test at 30Vin|15Vout, and the only difference is Case 2 is 2.33A while Case 3 is 1.5A. 

    The slow start sounds like soft-start where the output voltage slowly ramps up to final value once the device is properly switching.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    TI__Intellectual 2690 points

    Hi Jimmy,

    Yes, in case 3), we tried at lower load (1.5A) and TPS5450 can restart successfully. Here I don't understand.

    about the slow start, is its working principle the same as soft-start? Does TPS5450 has soft-start during powerup? I don't see description about soft start in its datasheet. 

    Best regard,

    YANG

  • 0 in reply to Yang Wu
    TI__Mastermind 41760 points

    Hi Yang,

    Yes the TPS5450 has an internal soft-start during startup/power up. 
    Once the enable voltage is above roughly 1.2-1.3V, the part will begin switching and output voltage will climb to final value within 8ms.

    Regards,

    Jimmy