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TPS62290: Increasing Soft-Start time

BerndM
Prodigy 40 points
Part Number: TPS62290
Other Parts Discussed in Thread: TPS62000, TPS82084, TPS62097,

I have tried to increase the soft-start time (RCD) element, but my chosen value aren't good.

Is there any appnote particular for this series ?

Background is that we need a USB-bus powered device.

Or are there any better regulator for that purpose (e.g TPS62000)?

thanks

Bernd

  • 0
    TI__Guru*** 142580 points
    Both devices you mentioned are very old. How about using a device with a SS pin, like TPS62097 or TPS82084?
  • 0 in reply to Chris Glaser
    Prodigy 40 points
    Our target is an output voltage of 3, upto 500mA
    But the design is restricted in pcb space. This will also limit the max inductor size.

    For USB-bus powered we need to restrict the current. TPS62097 would not meet the spec.
    Do you know any regulator which was used before for USB? TPS62000 has got a USB reference in the datasheet.
    But I'm wondering if this really means for USB-bus powered (USB 2.0).
    thanks
    Bernd
  • 0 in reply to BerndM
    TI__Guru*** 142580 points
    Yes, all these TPS62 regulators are frequently used for USB applications.

    I think you have a USB input voltage, and want 3Vout at 500 mA? What SS time do you need?
  • 0 in reply to Chris Glaser
    Prodigy 40 points
    Correct, 5 V input voltage. In the startup phase the current consumption will be low (3mA).
    I have not restriction regarding the SS time. But mainly the current will be defined by the inductor (limited to 15uH) and switching resistance.
    In my understanding of the USB spec, every mA above 100mA will be counted for the inrush current.

    Bernd
  • 0 in reply to BerndM
    TI__Guru*** 142580 points
    Sorry, but I do not understand your response. Why does the TPS62290 not work?

    Yes, the USB spec has various stages of current limit but this may not necessarily be relevant to every USB-powered device.
  • 0 in reply to Chris Glaser
    Prodigy 40 points

    According to the USB Peripheral compliance checklist, a peripheral device must limit the inrush current.

    It is not an average current. Every uC above 100mA will be counted (limit 150uC after waiver).

    During the SS the Coil/input current will be above 100mA (12uH) and so it will violate the USB spec.

  • 0 in reply to BerndM
    TI__Guru*** 142580 points
    Ah, ok. You need to limit the inrush current for the first 100 msec. One method would be to put a very large RC delay from Vin to the EN pin to delay the turn on of the DC/DC for 100 msec.

    Or you can use a DC/DC with a SS pin, like TPS62097, to reduce the inrush current.

    You might start a post in the power interface forum about how this USB spec is typically met by the peripheral.