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LM2678-ADJ DAC or PWM controlled output

Lee Combs
Prodigy 60 points
Other Parts Discussed in Thread: LM317, LM2678, LM10010

Hello,

I've been trying to configure the LM2678-ADJ simple switcher to be either controlled by a PWM or DAC signal. The schematic in the picture is the circuit I'm currently using to do this with. I have tried feeding the DAC signal in through the GND side of R6(1)1 but the reference voltage of 1.21V will increase to about 6V and the LM2678 dies. I got this idea from how the LM317 output can be controlled by changing the GND reference for its resistor divider.

I decided to try a summing method which is what you see here in the schematic. The idea is that as the voltage from the DAC or PWM increase the output decrease. This seems to work to a point, but then stops as described in the comments on the schematic. I'm having trouble trying to understand why I'm getting this behavior and solving the problem. Any help on the matter would be great. If you need further information please don't hesitate to ask I'll do my best and respond as soon as possible. I look forward to hearing from you.

Regards,

Lee

Spec info: Switcher input is around +33V DC, Op-amp rails +12V DC, -12V DC, and MCU is powered by +3.072V DC

  • TI__Mastermind 19101 points
    Hello Lee,

    Apologies for the late reply. Let us know if you were able to resolve this or if you still need help.

    By the way, have you considered using a device like the LM10010? It is a 6-bit current DAC that can be used to modify the output of a switcher.
    www.ti.com/.../LM10010

    Cheers,
    Denislav
  • in reply to Denislav Petkov
    Prodigy 60 points

    Hello,

    I've been patient.

    I still haven't solved it, but you gave some insight on the switcher. I thought about using a variable voltage control, but I didn't consider using an adjustable current to control the output. The LM10010 you mentioned is a current DAC. Would it be safe to assume that any adjustable current source applied to the feedback pin would work? Or is there something special about this IDAC that compliments a switching regulator?

    I'll experiment with this assumption. I may not be able to get the results to post till the weekend of 02/13/2016.

    Regards,

    Lee

  • in reply to Lee Combs
    TI__Genius 11085 points
    Yes, any adjustable current source applied to the FB pin should work. The amount of vout adjustment is isource*Rfbt. Current going into the FB node will reduce the Vout. The response time depends on the loop speed. -Yang