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TPS7A8300: About PSRR spec of use bias pin and bias pin open

Tsukui Yusuke
Guru 21930 points
Part Number: TPS7A8300
Other Parts Discussed in Thread: TPS7A83A

Hi,

Please let me talk about PSRR characteristics of TPS7A8300.
Which of using BIAS power supply and not using BIAS power supply has higher PSRR?
(In the case of TPS744, PSRR was higher when using Bias.)

If using bias is better,
Could you tell me the reason or background?

Best Regards,
Yusuke/Japan Disty

  • 0
    TI__Guru 50170 points

    Hi Yusuke-san,

    Only if VIN is >=1.4V is the use of Vbias optional. Benefiting from it will be a function of your Vin/Vout in your application.

    When Vbias is not present, the TPS7A83 has an internal charge pump (voltage doubler) so if Vin is 1.4V, the gate drive circuit is running at 2.8V. In this case, adding Vbias can help PSRR as adding either 3.3V or 5V gate drive gives us more headroom for controlling the pass FET. Figure 3 from the PDS shows this performance:

    So if Vin is below 2.2V, adding 5V on VBias can help to improve PSRR this is described a little in the absolute maximum ratings section of the datasheet.

    If Vin is higher than 2.2V, then the gate drive has sufficient headroom to drive the gate with optimum performance. Not if Vout is approaching 5V we run into the gate-drive headroom at the upper limit. This is shown in figure 20 & 21 of the datasheet:

    Does this answer your question?

    Regards

  • 0 in reply to JCHK
    Guru 21930 points

    Jhon-san,

    Thank you for your kind support.
    I will tell the customer the your explanation.

    Let me ask you one more thing.
    Customers are concerned about the quiescent current of the LDO.
    Which of using BIAS power supply and not using BIAS power supply has lower quiescent current?

    For example:
    1.Vin=1.5V,Vbias=open,Vout=1.2V
    2.Vin=1.5V,Vbias=3.3V,Vout=1.2V

    I want to know the quiescent current difference of 2 patterns.

    Could you give me your advice?

    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Guru 50170 points
    Hi Yusuke-san,

    I will have to confirm with the design team, but I do not believe that there is a big change in total Iq when using Vbias or not. The reason is that if present the Bias rail powers the internal circuitry as well as the gate drive. But let me confirm.

    Regards,
  • 0 in reply to JCHK
    TI__Expert 4950 points

    Yusuke-san,

    The overall quiescent current is the same.  If a BIAS rail is used, the BIAS rail will have to supply the 2.3mA of the internal circuit.  If the BIAS rail is not used, the current will be pulled from the IN rail.  The total quiescent current is referred to as the GND pin current.

    You may want to look at the updated TPS7A8300, the recently released TPS7A83A that has a more complete characterization of both PSRR and noise under various conditions.

    Best regards,

    Xavier