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I am trying to set under voltage, over voltage and over voltage clamping limits in a circuit for Overvoltage Output Regulator with Reverse Current Protection using LTC4364 device. The LTC4364 uses an external resistor voltage divider to set the over, under voltage and overvoltage clamping(FB pin) limit set points.

In the datasheet they have some example circuits with OV, UV and Overvoltage clamping limits with Resistor voltage divider but i am not able to understand how they calculated resistor values for certain OV and UV limits.image1 Image2

with Vin=12V and to set UV to 6V they have used 383K(R1) and 100K(R2) as well to set OV to 60V they have used 475K(R1) and 10K(R2). I am not able to get these values with basic voltage divider fromula.

My question is I want to set UV to 18v and OV to 32 V and overvoltage clamping to 35V at FB pin with Vin as 24V.Can anyone help me how to calculate resistors values for desired set limits.

The LTC4364 data sheet is here LTC4364

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The UV and OV inputs trigger at about 1.25 volts (read the data sheet) so, using R1, R2 and R3 values from your top digram, you get a potential divider ratio at the UV input of: -

(90.9 + 10)/(90.9 + 10 + 383) = 0.2085

So if the input voltage is 6 volts you get 1.25 volts at UV.

Note that your top diagram has "6 V" written beside the UV pin. It doesn't mean that 6 volts appears at that pin, it means that when Vin is 6 volts or below the UV lockout circuit triggers.

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  • \$\begingroup\$ Ok, Thanks. If i want to set UV limit to 18V then i should choose resistor values at voltage divider, (R2/R1+R2)= 0.0694, then only i can get 1.25 at UV. \$\endgroup\$ – verendra Jul 18 '18 at 12:07
  • \$\begingroup\$ Figure 1 has R3 that needs to be taken into account but, if R3 is zero ohms, then the potential divider ratio is 1.25/18 = 0.0694. \$\endgroup\$ – Andy aka Jul 18 '18 at 12:12
  • \$\begingroup\$ My personal experience on both LTSpice simulation and prototype: OverVoltage pin is useless. Output is clamped at the point set by FB resistors. Everything else works normal. Vout goes down after Delay_Time + Warning_Time and comes back after Cool_Down period. \$\endgroup\$ – Mohsen Jan 23 '20 at 19:51

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