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how to adjust the output voltage of a buck/boost controller via a microcontroller (DAC, PWM, SPI,...)?

Luc Tronco
Prodigy 50 points
Other Parts Discussed in Thread: UCD3138, LM25011, UCD9240, LM10011, LM25118

Hi,

I want to make a variable power supply controlled by a microcontroller (stm32f767zi).

The input voltage will be 24V. The output voltage should be adjustable (via the microcontroller) between 14V and 31V.

The output current is maximum 0.8A.

What kind of IC can I use and how can I adjust the output voltage via the microcontroller (DAC, PWM,SPI...)?

I can, if needed, boost the 24V first to for example 35V with a seperate (boost controller) IC and use this voltage for the input voltage of a buck controller Ic.

How can I adjust the voltage of this buck controller IC via our microcontroller? The microcontroller has DAC, PWM, SPI output.

Best regards,

Luc

  • 0
    TI__Mastermind 18370 points
    If you have firmware writing capabilities, then you can use one of our UCD3138 digital controller to do this.
    Your ST microcontroller can send a command to our UCD3138 through SPI or I2C and ask for the desired output voltage.
    UCD3138 can also switch from Buck mode of operation to Boost, this way you save the pre-boost stage.
    What are the quantities involved? How soon you need this?
  • 0 in reply to Yitzhak Bolurian
    Prodigy 50 points

    Hi Yitzhak,

    thank you for the information!

    I have seen from the specifications of the inkjet head, I also need 39V (and 5V).

    So in total I need 1 time 5V, 1 time 39V and 2 times adjustable voltage from 14V to 31V for each board.

    How can I use the UCD3188 to make these voltages?

    I need probably 100 boards a year.

    An other solution is boost the 24V input voltage to 39V (I need TDS40210 ->webench) and use 2 LM25011 for the adjustable voltages together with a 2 digital potentiometer ( AD5270) or use 2 DAC of the ST microcontroller.

    What do you suggest?

    Best regards,

    Luc

  • 0 in reply to Luc Tronco
    TI__Mastermind 18370 points
    Hi Luc,

    Based on your description, I realize that your request is more suitable for "Non-Isolated DC/DC Forum ".
    In the non-isolated DC/DC group in TI there are several solutions that do not require firmware development.
    The firmware is already available and the device can be configured through a GUI.
    Device such as UCD9240 and UCD9xxx are develped by the non-isolated group.
    I will try to forward your question to Zhiyuan Hu.
  • 0 in reply to Yitzhak Bolurian
    TI__Genius 9845 points

    Hi Luc,

    It appears to me LM10011 + LM25118 would be an option, through I haven't looked into the details. If your MCU has a DAC output, it can directly adjust LM25118 voltage like LM10011 does. You can look into LM10011 datasheet for details.

    Regards,

    Zhiyuan

  • 0 in reply to Zhiyuan Hu
    Prodigy 50 points

    Hi Zhiyan,

    thank you for the information!

    I have checked the datasheet of the LM10011 an I can't use it because it has only 6 bit resolution and a current span of 56.4μA.  I have to be able to adjust the voltage from 14V to 31V in steps of 0.1V.

    The ST microcontroller, I'm using, has 2 X 12 bit DAC. The minimum resistive load of the DAC is 5kOhm. This minimum resistive load is probably too high (maximum output current of the DAC is too low ) to have a output voltage range of 14V to 31V.  Do you think a DAC with external buffer is a better solution than a digital potentiometer?

    Best regards,

    Luc

  • 0 in reply to Luc Tronco
    TI__Genius 9845 points

    Yes. You can use an external buffer.

    You can also increase the feedback resistor divider value to reduce the current needed from the DAC.

    Or do both.

    Regards,

    Zhiyuan

  • 0 in reply to Zhiyuan Hu
    Prodigy 50 points
    Hi Zhiyan,

    in the datasheet they say the resistor value of Rfb should be between 1k Ohm and 10 kOhm.
    Can I use a higher value resistor?

    Best regards,

    Luc
  • 0 in reply to Luc Tronco
    TI__Genius 9845 points

    Luc,

    I believe 10k or 20k doesn't make much difference. What matters is the upper resistor value of the resistor divider. Say you want to change Vout by 10V, with 10kohm upper resistor, the current needed from the buffer is 10V/10kohm=1mA. It shouldn't be a problem if you use external buffer.

    Please refer to Section 2.1 of the app note below:

    Regards,

    Zhiyuan 

  • 0 in reply to Zhiyuan Hu
    Prodigy 50 points
    Hi Zhiyuan,

    thank you for your help!
    Maybe I will use a pwm signal instead of a DAC to control the voltage . With the PWM I don't need a buffer (just RC network).

    Best regards,

    Luc