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schematic

simulate this circuit – Schematic created using CircuitLab

In above diagram feeding 0-5V on inverting input of opamp will vary the output of LM2596, The problem is R1 is getting really hot at 20V.

Yes of course I can increase the R1 and R2 values to 10K and 3.3K but in that case if we close the SW1 it will take several seconds until LM2596 output become ~0V.

Why increasing the R1 and R2 values causes slow voltage drop when SW1 is closed and how to achieve instant 0V when SW1 is closed.

*I don't want to use ON/OFF pin to hit 0V.

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The problem is that you are using your voltage divider also as the "bleed" resistor for your output filter capacitor. When you stop the regulator from operating, this capacitor needs to be discharged, and that takes time.

To make it happen faster, you either need to switch to a smaller capacitor, or you need to provide a separate path for that discharge current.


Or, as I said in a comment, it might be better to block the current rather than shorting it out. As long as it's inside the feedback loop, it won't materially affect output regulation. For example, use a PNP transistor fed by a current sink:

schematic

simulate this circuit – Schematic created using CircuitLab

Q1 needs to be rated for the maximum voltage and current of the power supply.

Q2 needs to be rated for the maximum voltage, and it needs to sink enough current to keep Q1 saturated when on.

Select R2 based on the control logic voltage and the current required for Q1.

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  • \$\begingroup\$ Corrected the schematic, thanks. What's the best way to provide a path for capacitor to discharge? should I add a 10mA current sink or an LED or is there better ways to do it? \$\endgroup\$ – ElectronSurf Nov 14 '19 at 12:40
  • \$\begingroup\$ That depends -- how fast do you want the voltage to fall? How big is the output capacitor? How much power can you afford to waste? Maybe it would be better to put something in series with the output to block the voltage rather than discharging it. \$\endgroup\$ – Dave Tweed Nov 14 '19 at 12:43
  • \$\begingroup\$ The capacitor is 470uF, I want it to snap to 0V and it's better not to waste any power but it's okay if we have to. What do I put in series with capacitor? a resistor? \$\endgroup\$ – ElectronSurf Nov 14 '19 at 12:52
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    \$\begingroup\$ A switch or transistor. As long as it's inside the feedback loop, it won't materially affect output regulation. \$\endgroup\$ – Dave Tweed Nov 14 '19 at 13:00
  • \$\begingroup\$ I'm really sorry, but how do I put the transistor there? a PNP transistor? how to I run it's base? \$\endgroup\$ – ElectronSurf Nov 14 '19 at 16:23

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