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LMR33640: Current injection of FB pin with a voltage DAC

Roy van Lierop
Prodigy 80 points
Part Number: LMR33640

Hi,

I would like my math to be reviewed as a mistake is easily made.

My goal is to have a voltage DAC control the LMR33640 between +8V and +24V with an input voltage of 24.3V. The DAC can output between 0 and 2.048V and can sink/source 25mA max.

I'll set my default voltage halfway at 15.7V and then inject or source current with the voltage DAC. This default voltage output is achieved with a 100k resistor (RFBT) and a 6k8 resistor (RFBB).

By adding a 10k resistor to GND we get an output voltage of 25.7V, this can be done with a voltage DAC set to 0V and a 10k resistor. Not sure how to add negative resistors together, can I just assume it's linear?

Thus applying 2V through a 10k resistor results in a voltage of 5.7V?

(Vref = 1.0V)

15.7V = ((100000Ω / 6800Ω) +1)*1.0V

25.7V = ((100000Ω / (6800Ω^-1+ 10000Ω^-1)^-1 +1)*1.0V

5.7V = inverse of above?

A 12bit DAC yields me about 3250 steps between 8.0V and 24V.

  • 0
    TI__Expert 3400 points

    Hi Roy,

    Your calculations appear correct regarding the top resistor R_FBT and bottom resistor R_FBB with the corresponding output voltages 15.7 and 25.7 Volts. 

    Please share more details for your schematic and output current to address how you expect to achieve 5.7 Volts or how you apply 2 Volts across the 10k resistor.

    I hope I addressed all of your questions.

    -Ryan

  • 0 in reply to Ryan
    Prodigy 80 points

    Hi Ryan,

    Here is my schematic:

    I'm using the MCP47CVB28 quad voltage DAC to set the voltage of one side of the 10k resistor. Setting it to 0V will sink current into the DAC, setting the output to 25.7V. Setting the output to 2V will source current from the DAC, hopefully outputting 5.7V.

  • +1 in reply to Roy van Lierop
    TI__Expert 3400 points

    Hi Roy,

    Your method looks correct. The DAC appears to be able to sink/source enough current for FB injection with a 10k resistor.

    Let me know if you have any follow up. I hope this answered your question.

    Regards,

    Ryan