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UVLO=Under voltage Lock out

I have a circuit made in LTspice which is straight from the datasheet of the LT8315 flyback converter. It takes an input voltage range of 20V-450V and outputs a constant output voltage of 12V. Currently, the UVLO pin is simply connected to bias as shown in the image below:

enter image description here

LT8315 Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/8315fa.pdf

I am trying to use the UVLO pin such that UVLO functionality occurs when the input voltage is less than 60V. The datasheet says to connect the UVLO pin to a resistor divider if UVLO functionality is desired.

According to the datasheet, the chip will not operate if the voltage at the UVLO pin is less than or equal to 1.22V. So, when the input voltage is exactly 60V, I want a resistor divider such that the voltage at the UVLO pin is exactly 1.22V. I have done the calculations for that below:

Vout=(R2/(R1+R2))Vin 1.22=(R2/(R2+R1))60

0.00203333=(R2/R2+R1)

Let R2=100ohms

Then, R1=(0.998/0.002)R2 R1=(0.998/0.002)(100ohms)=49.990kohms

Essentially, my question is how exactly and where exactly would I connect the resistor divider so that I can accomplish my goal?

I am sorry if this is a bit of a dumb question but I'm figuring this out as I go. If anyone has experience with flyback converter circuit design, I would appreciate your advice as I am a bit confused on how exactly to connect the resistors.

Thank you!

Also, here is a download link to the LTspice schematic. It's just below. Let me know if it does not work for you.

https://file.io/Z6s2KsQGQIx7

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1 Answer 1

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enter image description here

Like this. The UVLO divider can't be placed other than that.

1.22/60=0.02, so the correct resistor value is 10x smaller resistance.

$$\dfrac{V_{out}}{V_{in}}=\dfrac{1.22}{60}=k$$ $$\dfrac{R_2}{R_1+R_2}=k$$ $$R_2=R_2\cdot k +R_1\cdot k$$ $$\dfrac{R_2(1-k)}{k}=R_1$$ $$R_1(R_2=100\Omega)\approx 4,8k\Omega$$

Of course practically these values are wrong, since there would be lot of dissipation heat.

enter image description here

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  • \$\begingroup\$ Thank you Marko for putting me on the right track with your response. I made this circuit as in the photo you included. Where R1=4.8K and R2=100ohms for the resistor divider. However, The UVLO functionality still does not work as intended. The output voltage is 0V regardless of whether the Vin is set to 35V or 350V. I assume this has to do with the high power dissipation for R1 at high voltages? In this case, how can I get the simulation to work as intended? If you are not 100% sure, perhaps you could point me in the right direction(i.e what I should read/further research). Thanks. \$\endgroup\$
    – mstar42
    Commented Sep 27, 2021 at 4:22
  • \$\begingroup\$ @mstar42 it works for me, see the green line. \$\endgroup\$ Commented Sep 27, 2021 at 7:33
  • \$\begingroup\$ Nevermind, I got it. Thank you! \$\endgroup\$
    – mstar42
    Commented Sep 28, 2021 at 15:04

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