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I built up a circuit which provides power for raspberry pi and its other peripherals based on LM2596. System is powered by 12V 7Ah VRLA battery and my problem is when voltage is at a certain point (like 11.85v) LM2596 starts to turn on/off frequently. (I fried up one raspberry because of this problem :( ) Can anyone help me about this problem? enter image description here

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  • \$\begingroup\$ (Question has been reopened after the OP added their actual schematic, instead of a copy from a datasheet which didn't match their actual usage.) \$\endgroup\$
    – SamGibson
    Jul 16, 2023 at 17:40

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You have a 12V zener.

If you for a moment assume it is an ideal zener which conducts current when there is 12V over it, and does not conduct when there is less than 12V over it, your zener will only conduct and turns on the transistot when there is more than 12V on input.

If that's not what you want, and need to work with input voltages below 12V, use a zener with voltage less than 12V.

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  • \$\begingroup\$ If there is 12V on the Zener..... There needs to be 0.65V across R24 and another 0.65 across R25. You need positive feed back on Q2. What happens when the battery is low, load current drops the voltage some, then Q2 turns off the PWM which releases the load current. Now with no load the battery voltage jumps back up, turning the PWM back on. (oscillation) You probably need to turn off the PWM when battery voltage is under 11.5V and turn the PWM back on when the voltage is above 12V. That 0.5V difference will stop the oscillation. \$\endgroup\$
    – user338146
    Jul 16, 2023 at 18:25
  • \$\begingroup\$ Yes exactly. Here comes the problem. That 0.5V hysteresis is my need but its not working ... \$\endgroup\$
    – ilker_tr
    Jul 16, 2023 at 18:32
  • \$\begingroup\$ In addition to this answer, you do not show any high frequency power bypass capacitors. PC board layout is also important, and an overvoltage crowbar for the processor logic supply would prevent the fried Raspberries. \$\endgroup\$
    – PStechPaul
    Jul 16, 2023 at 20:42

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