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Original question:

TPS65381 VDD1sense output current

TPS65381A-Q1: VDD1 Output Current and divider resistance

Kian Lin
Intellectual 1710 points
Part Number: TPS65381A-Q1

Hi Team,

I have a question about VDD1 Output Current and divider resistance, could you kindly ask below questions:

  • What's 'IVDD1 Minimum current realized with external resistive divider' mean? 10mA current will limit what configuration? When should we consider 10mA minimum current?
  • Why VDD1_SENSE pin must to be set to 800mV? Why we need sufficient current? Is 80.6 resistance is a fixed value? Can I choose other value like 3k?

 

I don't know why this picture is so flurry, you can find this in page 11 and 97 of TPS65381A datasheet.

Thanks

BRs

Kian

  • 0
    TI__Expert 6700 points

    Hello Kian,

    Kian Lin said:
    What's 'IVDD1 Minimum current realized with external resistive divider' mean? 10mA current will limit what configuration? When should we consider 10mA minimum current?

    The main reason for the 10mA current for VDD1 is stability in the regulator. Anything below 10mA, the regulator functionality cannot be guaranteed which leads into your next question below.

    Kian Lin said:
    Why VDD1_SENSE pin must to be set to 800mV? Why we need sufficient current? Is 80.6 resistance is a fixed value? Can I choose other value like 3k?

    As stated in the datasheet, a value of R1 should be ~80.6ohm to ensure a stabilizing current is maintained.  A higher R1 value will result in a lower current which might lead to instability in the regulator.

    Regards,

    Alex

  • 0 in reply to Alex Saenz
    TI__Intellectual 1710 points

    Hi Alex,

    Thanks for your answer. I also have a question about why does VDD1 lead to instability when IVDD1<10mA.

    I run VDD1 simulation, VIN=1.2V and set VDD1 as 1.1V (divider resistance=3k) with no load. I didn't see instability from VDD1 waveform.

    Thanks

    BRs

    Kian

  • 0 in reply to Kian Lin
    TI__Expert 6700 points

    Hello Kian,

    The available simulation models for this device may not be taking into account the internal biasing that is required to ensure this stability. The value of 80.6ohm is also stated in the following section of the datasheet.

    Regards,

    Alex

  • 0 in reply to Alex Saenz
    TI__Intellectual 1710 points

    Hi Alex,

    Thanks for your reply. But if simulation can't verify this instability, which test condition can I use to verify this in my prototype(I have already used 3k resistor divider) ?

    And I also want to ask why this resistor divider need 10mA bias current? How to analyze this from principle level?

    Thanks

    BRs

    Kian

  • 0 in reply to Kian Lin
    TI__Genius 12735 points

    Hi Kian,

    The model can be used to determine if the ratio between the resistors is correct for the chosen output voltage. However the model does not take into account the bias current required for the feedback pin (VDD1_SENSE) to function properly. Without proper bias current the output voltage from VDD1 will not be correct based on the resistor divider ratio.

    More information about calculating these values can be found in section 6.2.2.2 of the datasheet. 

    Best regards,

    Layne J

  • 0 in reply to Layne J
    TI__Intellectual 1710 points

    Hi Layne,

    Thanks for your reply.

    This VDD1_SENSE  bias current is just 10uA. When we set resistor divider to 80.6ohm, the current flow through this path is 10mA. So I am confused about If we need so large current in resistor divider path. 

    And I also find IVDD1 minimum value is 10mA, so I want to confirm with you if we set 80.6ohm is to satisfy 10mA minimum current. And why IVDD1<10mA will cause instability.

    In my design, VDD1 is set to 1.1V, We can't choose a 30.225ohm resistance when low-side resistor is set to 80.6ohm . So could I use a 3kohm divider and add a dummy load to satisfy minimum IVDD1 current?

    Thanks

    BRs

    Kian

  • +1 in reply to Kian Lin
    TI__Genius 12735 points

    Hi Kian,

    The feedback resistor values are required to be around or below 100 ohms due to the amount of current required to bias the internal feedback comparator, not due to a regulator output minimum requirement. When a resistor of a higher value is chosen then the internal comparator will not be properly biased and the DC output regulation of VDD1 will not be accurate. 

    The model available does not take this into account but this is mentioned in section 6.2.2.2 of the datasheet and this must be taken into account to get an accurate output from VDD1. 

    Best regards,

    Layne J

  • 0 in reply to Layne J
    TI__Intellectual 1710 points

    Hi Layne,

    Got that. Thanks for your explanation. I will choose a suitable standard resistance. Thanks

    BRs

    Kian