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DAC7574: Designing with DAC7574IDGS

Shibin Varghese
Intellectual 835 points
Part Number: DAC7574

Hi Team,
I am using DAC7574IDGS in one of my projects.
The same is used to injust an analog voltage to the feedback pin of a buck/boost converter in order to adjust the output voltage of the buck or boost converter.
I need to use both 4 channels of the DAC.
1). Can I get 4 different voltages at the same time from different channels?.
2).  Why do we need an RC low pass filter at the output of the DAC?.
4). What value should I choose for the RC filter?.
5). Attached is the schematic of the same. Please let me know if there are any mistakes.

  • 0
    TI__Mastermind 19086 points

    Hi,

    Please find the answers below in order.

    1. Yes, you can get  4 different voltages from 4 different channels by following method.

    write into temporary registers for all DAC channels and then issue broadcast command to update all the channels

    2.  Not necessary. Adding RC filter will reduce the noise from the DAC output channels. Values in the schematics looks fine

    Schematics looks OK.

    Regards,

    AK

  • 0 in reply to Akhilesh K
    Intellectual 835 points

    Hi Akhilesh,
    Thank you for your reply.
    The output of the DAC should remain in a high impedance state when the buck or boost converter is not operational.

    1). Could you please explain the difference between high impedance output and high impedance referring to the datasheet?
    2). Also, is it possible to leave the channels individually in the high impedance state?.

  • 0 in reply to Shibin Varghese
    TI__Mastermind 19086 points

    Hi,

    High impedance DAC output means, VOUT node is disconnected from DAC. In your application, high impedance is needed so that at power up, buck converter will be t nominal output voltage (since DAC is disconnected).

    Yes, you have individual channel options for power down with high impedance options. Please have a look at the datasheet.

    Also we have devices specifically for this kind of applications. See DACx3204

    https://www.ti.com/product/DAC53204

    App note explaining power supply margin with DACx3204

    https://www.ti.com/lit/pdf/slaae23

    Regards,

    AK

  • 0 in reply to Akhilesh K
    Intellectual 835 points

    Hello Akhilesh,
    Thank you for your reply.
    We changed the DAC to DAC53004RTER.
    1). Attached is the design of the same. Please let me know if there are any mistakes


    2). We want voltage at the output of each channel. so I believe the VOUT and the corresponding FB pins should be shorted together.
    3). Since this IC has a current output as well, what is the minimum current we can get from this IC?.
    4). If I just want to use only 3 of its channels, are there any termination methods I should follow for this IC?.
    5). Just for curiosity, can I adjust the gain by adding a resistor between the OUT and FB pin?. Because the BUF will act as a non-inverting amplifier using the external resistor, R1, and R2

  • 0 in reply to Shibin Varghese
    TI__Guru 53573 points

    AK will get a response to you tonight.

  • +1 in reply to Shibin Varghese
    TI__Mastermind 19086 points

    Hi,

    Here are my comments.

    1. Do you plan to use external reference or internal reference? I suggest go with internal reference for better accuracy.

    2. LDO cap max is 1.5uF (pin 13)

    3. REF cap should be limited to 0.1uF

    4. You need to short FB and VOUT for proper operation. For current output mode operation, leave FB pin as floating. current outputs can be set to following ranges :v ±25 μA, ±50 μA, ±125 μA, ±250 μA.

    5. No need to terminate any of the channels.

    6. Gain is set internally to account for the headroom requirements, so we dont suggest adding external resistor. In theory, yes you can adjust the gain by adding external resistor.

    Regards,

    AK

  • +1 in reply to Akhilesh K
    Intellectual 835 points

    Hello AK,
    Thank you for your reply.
    The internal reference of the DAC53004RTER is 1.212V.
    By adjusting the gain, I hope I will be able to get a voltage up to the supply voltage.