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DAC53701: Slew Rate Control

Part Number: DAC53701

Hi Team,

We are currently using the DAC53701 to connect to the FB pin of a DCDC converter, allowing us to control the output voltage of the DCDC digitally. One item we are running into issues with is controlling the slew rate of the DCDC through the slew rate of the DAC. We are trying to configure the slowest possible slew rate setting but are not seeing it take effect. Have any recommendations as to what could be causing the issue or when to look? 

  • Hi Adam,

    The slew rates given in table 8-4 are used when the DAC output is being switched from the value written in the MARGIN_HIGH and MARGIN low registers through a software trigger or the GPI hardware trigger. To get around this, you can write the intended values to the MARGIN_HIGH or MARGIN_LOW register, and then write a SW trigger in the PMBUS_OPERATION Register:

    Write 0xA4 to trigger margin high, and 0x94 to trigger margin low. The device doesn't need to be set in PMBUS mode to use this register. Margin high needs to be greater than the value in margin low or the slew feature will not wok correctly. So when triggering a margin high command, the new value must always be higher. If you have already triggered a margin high value, you can re-write a new value into the margin high register and trigger a second margin high command. If the value is lower then you would write the new value into margin low and then trigger a margin low command. 

    The slew rate feature was not designed to be used with the DAC_DATA register, so this is the only way I've found to get around that.

    Let me know if you have any additional questions. 


    Katlynne Jones 

  • Hi Katlynne,

    Thank you for the quick detailed response, this definitely helps. 

    One additional question I have is if we dynamically change the Margin High value when the DAC is currently outputting the margin high value. Will we need to resend the 0xA4 write to register 0x01 to have the DAC output the new value, or since the margin high is already triggered, will writing the new value to 0x25 trigger the new output value? 

  • Hi Adam,

    Yes, whenever you write a new value to the margin high or low register you'd always write the corresponding trigger to register 0x1, even if it is a duplicate trigger. So each update will require two commands. The output will respond to a margin high trigger as long as the new value in the margin high register is higher than the current output, and same for a margin low trigger as long as the new value in the margin low register is lower than the current output.

    I am not sure what the behavior would be if you change the margin high value and write a duplicate trigger while the output was already in the process of slewing. That is not something I've tested.