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TPS25221: The question for TPS25221 for control output current

James Chen
Prodigy 110 points
Part Number: TPS25221

I read the some data for TPS25221 as shown in picture, and I see
the function for " when output current to the current-limit threshold IOS during an over-current or short-circuit,
it will reduce the charge pump voltage driving the N-channel MOSFET and operating it in the saturation region",

I don't understand why it can let current keep at Ios?

  • 0
    TI__Mastermind 18031 points

    Hi James,

    As described in datasheet, it operates in saturation region which means the high conduction resistance to limit the current and leads to VBUS dropping. The root cause is due to the discharge of gate cap to a certain level.

    If you have any other concern, contact me any time.

    If you think resolved, please help click 'Issue is solved button'.

    Thanks.

  • 0 in reply to Ziv Zhang
    TI__Prodigy 110 points

    So, regardless the current>Ios or current<Ios ,the mosfet are always in saturation region,right ?
    But when current>Ios, the voltage for gate cap will decrease, cause Id current keep at Ios.

    For my conclusion, am I correct?

  • 0 in reply to James Chen
    TI__Mastermind 18031 points

    For the first line, as >Ios, works in saturation region. As <Ios, works in variable resistor region.

    For others, it's correct.

    If you have any other concern, contact me any time.

    If you think resolved, please help click 'Issue is solved button'.

    Thanks.

  • 0 in reply to Ziv Zhang
    TI__Prodigy 110 points

    Thanks for you reply, I can understand if >Ios, the N-Mos will work in saturation region, and through adjust the voltage for Vgs to keep current at Ios.
    But when current<Ios, it's in the normal operating, it will let Vout almost equal Vin right ? Is it mean the N-Mos will be acted as switch device? If yes, why the Mos is in resistor region and not in saturation region.

  • 0 in reply to Ziv Zhang
    TI__Prodigy 110 points


    And <Ios, According the explanation in datasheet, that sounds like the MOS is in saturation region.  
    The mos in this situation like the switch.

  • +1 in reply to James Chen
    TI__Mastermind 18031 points

    As <Ios, it's like a switch. The Rdson is small. Regarding the definition of saturation or variable resistor, please check related doc.