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TPS65150: Low Vref causing low Output voltages?

Part Number: TPS65150

Hello,

I am trying to use the TPS65150 to create 10V, -14V, and 18V all from one power supply using a 1.8V VIN. I have attached my schematic below. I followed the datasheet and SLVA794.pdf to create a double stage for the negative charge pump.

The power supplies come up, but not to the correct level. The boost converter comes up to 7.4V, and negative charge pump comes up to -11V, and the positive charge pump comes up to 14.5V.

I measured the part's internal Vref on pin 1 as 0.855 V, which should be around 1.2V according to the datasheet.

Do you have any suggestions for why Vref is not reaching the specified level? I suspect the low Vref is causing the rest of the supplies to be too low.

Thanks for the help!7752.TPS65150_schematic.pdf

  • Hello Benjamin,

    Thanks for your request. From where do you supply the 1.8 V? Please make sure that 1.8 V are measured directly on the VIN pin and not damped through the supply cable. What is the current limitation of your supply?


    Best Regards.
    Ilona
  • Ilona,

    I am making 1.8V on-board. I measured the VIN supply at pin 4 of the TPS65150 and it looks good. I've included scope grabs.

    EDIT: The 1.8V power supply is a 25A supply. The board is expected to draw 15A from the 1.8V rail apart from the TPS65150.

    Thanks for looking into this.

  • I've tried a couple things and here is what I have learned:

    I've played around with the inductor size, which has had no effect on Vref or any of the output voltages.

    I've played around with the external compensation for the boost converter, which moved Vref (on pin 1) up 10 mV, and adjusted the three output voltages slightly (from 7.4V to 7.5V on the boost converter.)

    I've disconnected the 1.8V VIN and tried a bench-supplied 3.3V VIN, which had no effect on Vref or any of the output voltages.

    Do you have any guesses as to why the Vref would be lower than specified?

  • hello Benjamin,

    Thanks for the update. This sounds quite odd at the moment, as the schematic looks okay. Can you measure if the resistor values are correct? We have seen that sometimes the components are either mistakenly wrong or placed not properly on the board...
    You can try to change the resistor divider to get another vout, what does it show now?

    Thanks.
    Best Regards.
    ilona
  • Ilona,

    After measuring the feedback voltages (0.85 V for the boost converter, 0.9 for the negative charge pump, and 1.02 for the positive charge pump), I calculated the resistor values needed to make my desired outputs based on these Vrefs. After putting those resistors in, the TPS65150 does make the voltages that I want, and the Vrefs stays at the previously measured values.

    We are still concerned that the Vref values do not matched those specified in the datasheet - will this impact part lifetime / will these Vrefs change over time / how will the part behave over temperature / other unknown issues?

    Regards,

    Ben

  • Hello Ben,

    Thanks for the update. Have you tried to replace the IC with another one to exclude that this is not related to the external circuitry?
    I do not think that it is a solid solution just to 'adapt' the resistor network to the VREF that you are observing.
    If you can confirm that this is only caused by this one IC this would be a case for a quality issue.

    Thank you.
    Best Regards.
    ilona
  • Ilona,

    We have solved one issue and now we have another : )

    We noticed in the typical applications section there is an AC filtering capacitor (C8) on between the REF pin and ground - see attached. Installing this capacitor on our board makes all the reference voltages correct! However, the supplies still drop once a load is added to the circuit. Increasing VIN to 3.3V from 1.8V in conjunction with C8 allows all supplies to come up to the desired values and stay stable once a load is applied.

    Our new problem: We were hoping to draw 50 mA from the positive charge pump at 18 V. As you can see from our schematic above, we do not pass this supply through the Gate-Voltage Shaping block. The supply is stable at 18 V with a 25 mA load, but drawing any more current from the supply causes the voltage to drop. Can you comment on the ability of the positive charge pump to make 50 mA without the use of the Gate-Voltage Shaping functionality?

    Thanks for the continued support! 

  • Hello Benjamin,

    Of course! How could I have missed this! I am glad that you found it yourself.
    The positive charge pump should be able to deliver up to 50 mA output current, please take a look at this thread for your reference.
    How much drop do you get on VGH when you increase the load? Is that in the range of hundreds of millivolt or rather some volts?
    You may consider adding a feedforward capacitor (~some pF) in parallel to R60 to improve the load response.

    Thank you.
    Best Regards.
    Ilona
  • Ilona,

    The feedforward cap didn't do much for us, but I may have found another solution.

    We have 12V on the board, and if I disconnect the anode of D8 from Vout_boost (10V) and connect it to the 12V power supply, the positive charge pump works as expected. Is there any reason you wouldn't recommend this?

  • Hello Benjamin,

    Thanks for the update. Just to confirm, you mean that if you supply VGH from another 12 V rail VGH keeps stable (no voltage drop) at a load of 50 mA?
    You should be okay doing that as it seems that you are hitting the limitation of the ouptut voltage capability of the charge pump.
    The maximum output voltage that you can get from a charge pump is 2xVout_boost - 2x VF(D8,D) - Vdrop, as these drops are dependent of the output current you probably hit the limit.

    Best Regards.
    ilona