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TPS7A8101: 1-1WM28K2

Part Number: TPS7A8101

Hi, My name is SANG-RAE LEE worked for IT-Telecom in Korea.

Main items of my company are V2X products including wireless network.

So we use power amplifier for WI-FI and TPS7A8101 is linear regulator to supply 5-volt power to power amplifier because it is low-noise, wide-bandwidth, high PSRR and low-dropout linear regulator.

The input power source of TPS7A8101 is step-down regulator.

The output voltage level of step-down regulator is 5 volts and the same with output voltage level of TPS7A8101.

We have some questions in this conditions.

     1) If the input voltage level of TPS7A8101 is equal to output voltage level, then what are the predictable problems?

              5V INPUT => [TPS7A8101] => 5V OUTPUT

     2) If the power amplifier current consumption of WI-FI is 1A, then what is the predictable output voltage level of TPS7A8101 including voltage drop or dropout voltage level?

              5V INPUT => [TPS7A8101] => 5V OUTPUT=>  [1A POWER AMPLIFIER]

     3) In another case, if we use 5.5 volts as input power voltage, then what is the predictable output voltage level of TPS7A8101 including voltage drop or dropout voltage level?

              5.5V INPUT => [TPS7A8101] => 5V OUTPUT=>  [1A POWER AMPLIFIER]

     4) What is the difference between case 2) and case 3)?

     5) Can you explain internal operation of TPS7A8101 in case of 2) and 3) above?

Thank you.

Good Bye.

Best Regards.

SANG-RAE LEE

  • Hi Sang-Rae,

    1) With Vin = Vout, the LDO is operating in dropout. The pass transistor/FET is fully on and a number of performance specs will be greatly degraded: PSRR, transient response for line (input) and load (output). You can assume PSRR is almost Zero for Vin = Vout. PSRR specs in the datasheet are listed with Vin = 4.3 V, Vout = 3.3 V, where headroom: Vin - Vout = 1 V.

    1 V is recommended headroom (Vin - Vout) for PSRR performance.
    Max dropout voltage is minimum headroom (Vin - Vout) for most other LDO performance specifications.

    For the TPS7A8101, I would recommend Vin = 6 V for datasheet spec PSRR performance in the Electrical Characteristics (EC) Table.
    At a minimum, Vin = 5.5 V for all other performance specs.

    Problems:
    - Vout will vary with Iout. As Iout increases, Vout will decrease because dropout is Iout dependent.
    - PSRR will be very low, and ripple/noise from the input step down converter will pass through the LDO with minimal attenuation. This will increase the phase noise and reduce sensitivity of the RF design in your Wi-Fi devices. As a result, error rates will go up and range will go down with your Wi-Fi devices.

    2) See the Electrical Characteristics Table in the datasheet for max dropout and output accuracy for Vout = Vin + 0.5 V. See Figure 5 for dropout voltage graph. We do not specify the LDO at Vin = Vout.

    3) Datasheet specifies output and dropout with Vin = Vout + 0.5 V. See the Electrical Characteristics Table.

    4) Case 2 LDO is in dropout. Case 3 LDO is operating in acceptable Vin range.
  • Hi, Eric.
    Thank you for your answer.
    We can understand about LDO operation and our predictable problems through your answer.
    Thank you.
    Good bye.


    Best Regards.
    SANG-RAE LEE