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Soft start feature in TPS55330

Michael Schwartz
Prodigy 50 points
Other Parts Discussed in Thread: TPS55330

Hi TI

 

I’m have a question regarding the Soft Start feature in TPS55330. To me it seems like the Soft start feature works very well first time TPS is enabled, but if TPS is disabled (EN = LOW) and the enabled again after say 10 secs, SS does not work as expected. To me it seems like the discharge feature when TPS is disabled is a bit strange. SS_cap is discharged fast to around 500mv, but then discharging stops there and voltage decays very slowly from here. It takes close to a minute for the SS_Cap to be fully discharge. Result is that if you power up the converter again within 10-20 secs after a power down (disable), then Soft start is not really working. Instead of limiting the inrush current to around 3-500mA in my setup, the peak inrush current is close to 1.5-2A. If I manually discharge the SS_cap before enabling the TPS converter, soft start works and inrush is limited but this is not the case if I power off, wait for 10-20sec. and then power on again.

 

A few words about the setup. I use a TPS55330_EVM-017. Input voltage is 3,7v from a power supply and output is adjusted to 5v. The output of the converter is decoupled with 2 x 1000uF/16v caps. These are required in my setup in order to deliver the peak load currents required. These caps are of cause part of the reason for the big inrush current but they are needed. I use a 1uF cap for soft start and I have nothing else connected to the SS pin.

 

So can you explain why the SS_cap is not fully discharged when TPS is disabled?

Do I have to add an external discharge circuit in order to ensure that SS works soon after a power off?

 

I hope that you can help me out.

 

Br Michael

  • 0
    TI__Mastermind 35165 points

    Hi Michael,

    This is related to how long it takes to discharge the 1µF SS capacitor. The 5kohm pull down only remains on for a fixed amount of time. If the SS cap is not discharged in this time it will only discharge through any leakage current.

    Since you need the 1µF cap in this case for limitng inrush current, I suggest adding an external discharge circuit. You could make it depend on the EN signal, such that when EN is low the discharge path is enabled.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 50 points

    Hi Anthony

    Thanks for your answer. It all makes sense then. I will add an external discharge circuit as you suggest. Then it will all work fine.

    BR Michael