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Variable Resistor clamping and output behaviour for LM2596-ADJ

Xenobius

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Hi,

I need to limit the voltage output of an LM2596 from 7v to 14V. According to the datasheet if R1 is 1k than I can put R2 as 4k7 to get ~7V and 10k3 for ~14V
Question 1: As per my attached schematic - this is how I solved it - is it correct? would you have clamped the resistance differently?

When the input voltage is not high enough, it is understandable that the regulator will drop some voltage and output whatever it has "available".
Question 2: Is this how it would work? I don't understand this bit.

resistor clamp.png

Thanks all
 

betwixt

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I've never seen a regulator that takes it's feedback from the input voltage!
I suspect R36 should go to ground instead of the input.

To get the voltages you need, find the values for the lowest voltage you want (7V) and a new value for the highest voltage (14V). Use a fixed resistor to set the lower limit and in series with it add a VR with value = the difference between low and high values. Adjusting it will then give a combined resistance of (fixedR + 0) to (fixedR + VR).

The formulas for calculating values are at the bottom of page 12 in the TI data sheet.

Brian.
 

Xenobius

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I cannot even copy a circuit from the datasheet! Glad you found an issue with it. Also thanks for clarifying because that's what I did, put R37 in series with RV1 as you suggested.

Thanks again
 

FvM

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I guess, the wrong R1 connection has been brought up by the unusual datasheet schematic layout.

LM2596.PNG

When the input voltage is not high enough, it is understandable that the regulator will drop some voltage and output whatever it has "available".
Yes. The datasheet has values for dropout (0.7 - 1.3 V) and minimal input (3.5 - 4V) voltage. Below minimal input voltage, output is switched off.
 

Xenobius

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Yeah I got confused but thankfully found an issue that was about to happen :)
Thanks for clarifying
 

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