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TPS65131 Soft start

Louis Diana
Intellectual 2180 points
Other Parts Discussed in Thread: TPS65130, TPS65131

How does the internal soft start work?

 

I have a customer that says the soft start is coming up in 100us instead of 1ms as stated in the data sheet.

  • 0
    TI__Guru**** 165766 points

    When each converter is enabled, the implemented switch current limit ramps up slowly to its nominal programmed value in about 1 ms.  Can your customer send scope plots similar to Figures 51 and 52 of http://focus.ti.com/lit/ds/symlink/tps65131.pdf on page 20?

  • 0 in reply to Jeff F
    TI__Guru**** 165766 points

    Based on review of the scope plot and comparison to bench testing and datasheet curves, the IC is soft-starting as expected.  There will be an approximate 1ms from EN going high to either rail starting to ramp.  Then the outputs will ramp slowly as the current limit increases.  Larger output capacitors or starting into a large load will increase the ramp time.  Note that the datasheet start up curves were from the TPS65130 with lower current limit so a TPS65131 will likely ramp faster.

  • 0 in reply to Jeff F
    TI__Intellectual 1880 points

    I was looking in the forum for an explanation about softstart of TPS65130/31 and found this post.

    Can you please confirm my understanding about softstart of TP65130/31 below?? And please correct me if I'm wrong. 

    Both outputs will starting to ramp at approximate 1ms after the EN turns high. Both outputs will ramp slowly as the current limit of both converters increase. start-up time for the converters to reach output nominal value is dependent on the output capacitor. As the current limit increase at the same rate, larger output capacitor results in a longer start-up time. TPS65131 ramps faster than TPS65130 since it has higher current limit.

    Soft start on TPS65130/31 is different from other boost converters which have precharge phase and fixed current limit phase during start-up. In TPS65130/31, at start-up the output voltage slowly increase as the current limit is increasing.

    Thank you in advance.

  • 0 in reply to Santhos Ario
    TI__Guru**** 165766 points

    Yes.  I reworded and consolidated.

    Soft start on TPS65130/31 VPOS rail is different from other TPS610xx boost converters which have precharge phase and fixed current limit phase during start-up. Both outputs start to ramp up/down the output voltage approximately 1ms after their respectively ENx pins goes high, with VPOS starting its ramp from VIN if the isolation switch is not installed.  Both outputs will ramp slowly as the current limit values of both converters increase.   The start-up time for the converters to reach  the output nominal value is dependent on the output capacitor; larger output capacitor results in a longer start-up time.  Even though VPOS and VNEG's current limits increase at the same rate, if the isolation switch is not installed, VPOS typically reaches regulation before VNEG since it starts ramping from VIN.  TPS65131 ramps faster than TPS65130 since it has higher current limit.