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TPS74801: how to know operate in Dropout mode?

Part Number: TPS74801
Other Parts Discussed in Thread: TPS720

Hello,

I saw the table1 of datasheet about work on dropout mode condition. I'm confusing about Vin, Ven & Bias description. I just would like to know:

1. does meet one of these condition of in-condition, the IC will be working on normal mode? or into one of modes that need to meet all of condition?

since my customer was set Vin=3.3V, Vout =1.8V, VBias=3.3V,  which mode will be into?

for this setting:

meet VIN>Vout+VDO(VIN), VEN> VEN(high), but Vbias not meet , Vbias is small than Vout + 1.6V. could you help to know which mode will be into?

  • Hi Mark,

    TPS74801 has a NMOS pass transistor with no internal charge pump. This LDO uses an external Bias pin to drive the NMOS pass transistor and provide very small dropout from Vin to Vout. When Vin = Vbias, this LDO does not have low dropout performance, dropout from Vin to Vout is much larger.

    Recommended Operating Conditions and Electrical Characteristics Table explain usage with Vin, Ven, Vbias:
    When Vin = Vbias, dropout is 1.31 V typical and 1.6 V max.

    In your design, Vbias - Vout = 1.5 V. Your TPS74801 is operating in Normal mode for typical conditions and possibly in Dropout mode since max dropout is 1.6 V.

    To get low dropout performance with TPS74801, Vbias ≥ Vout + 1.6 V. Then the max Vin to Vout dropout spec is 165 mV. Avoiding dropout also improves PSRR, output transient response, and quiescent current.
  • Thanks Eric,

    so it's not recommend to use on Vin=Vbias=3.3V when Vout=1.8V, right? if in dropout mode, the Vout is 1.7V when max dropout voltage is 1.6V, right? how much the current rating when into dropout mode?
    if the setting condition can't to change, do you have other solution to recommend for this setting ?

    Best Regards

    Mark Chen

  • Hi Mark,

    Operating in dropout is generally not recommended, because dynamic performance of the regulator is degraded. Yes, the Vout could be reduced to 1.7 V if the TPS74801 is operating with max dropout of 1.6 V.

    The current limit should not change in dropout, so you should be able to operate up to 1.5 A output.

    Regarding your last question, are you asking for another part number recommendation?
  • Hi Eric,

    Yes, do you have another part number suggestion to me ?
  • Hi Mark,

    Could you let us know what the current output requirement is? Without knowing this value, I can think of TPS720 which has a rating for 1.09V (typical) to 1.40V (max) for Bias drop voltage. This device should work better for your application. If this is enough current let us know, otherwise we can look into other products.

  • Hi Jose,

    The output voltage is 1.8V, do we have P2P solution to TPS74801?

    Best Regards,
    Mark Chen
  • Mark,

    I am trying to find  a solution for you here, but I would need to know what current output is needed. Also we offer the TPS74801 in VSON and VQFN, which one of these packages are you using?

  • Hi Jose,

    Same with TPS74801 current output should be fine. I was used VQFN currently.