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TPS551892-Q1: How to achieve 0.8 to 1.2V output

Nelson Downs
Prodigy 10 points
Part Number: TPS551892-Q1
Other Parts Discussed in Thread: TPS552892-Q1, TPS552892, LM61495-Q1,

Tool/software:

Datasheet says the output range is 0.8V to 22V,

But the VREF is 1.2V, so how can I achieve 0.8V output?

Specifically the datasheet calculates R2 = R1 / ((VOUT/VREF) - 1)

If we set VOUT < 1.2v, then R2 becomes negative?

I want to set VOUT to 0.85. How can I do this? Also relevant for TPS552892-Q1

  • 0
    TI__Genius 15020 points

    Hi Nelson,

    Thank you for reaching out. May I know what is the application here?

    For TPS552892/TPS51892, you can source current into FB to achieve 0.8V output.

    Also, you can use the I2C version TPS55189Q/TPS55289Q to set lower VREF and achieve 0.8V output without an external current source.

    Regards,

    Mulin

  • 0 in reply to Mulin Yuan
    Prodigy 10 points

    Thank you Mulin,

    I need 0.85V for VCCINT and MGTRAVCC on a Xilinx MPSoC. I'm working from a USB-C PD bus that's 5-20V

    I was planning on using the TPS551892-Q1 since I was using it for another 5V bus. I'm also using the LM61495-Q1 for several other low voltage busses (3.3V, 1.8V, 1.2V and 1.1V), but the LM61495-Q1 minimal output is 1V.

    So I was going to use TPS551892 for 0.85V, but ran into the issue. Is there another spread spectrum buck SMPS you recommend, or can you give some pointers on how to get 0.85V from TPS551892? I can source current from my 3.3V or 5V bus, but not 1.8V because of power supply sequencing

    Thank you,

    Nelson

  • +1 in reply to Nelson Downs
    Prodigy 10 points

    Hi Mulin,

    I think I see now how I can achieve this, with another resistor connected from 5V to FB

    With R1 from VOUT to FB, R2 from FB to GND, R3 from 5V to FB:

    I_R1 = (0.85 - 1.2)/R1

    I_R2 = 1.2/R2

    I_R3 = (5 - 1.2)/R3

    I_R2 = I_R1 + I_R3

    So if we use R1 = 100KΩ and R2 = 200KΩ then R3 = 400KΩ to get 0.85V on the output with 5V external supply

    Does this sound right? Do you have any further tips - e.g. filtering or using an LDO from 5V, or does it matter if I use the 5V directly from the other SMPS?

    Thank you,

    Nelson

  • +1 in reply to Nelson Downs
    TI__Genius 15020 points

    Hi Nelson.

    I think it is the right way to set the 0.85V output voltage. Notice that leakage current at FB pin is 100nA. So recommend to change R1,R2,R3 to 50kohm, 100kohm,200kohm to reduce the effect by the leakage current.

    The 5V source is determined by your requirement to Vout ripple / accuracy. Just make sure the 5V source is stable.

    Regards,

    Mulin