I have a modified LM5157EVM SEPIC with an LM51581 where the Bias if fed by the input and output through diodes (J6 1-2, J7 1-2). I would like a turn on at 6V, and a turn on at 2.24V. My UVLO resistor divider consists of a 68K and 22K based upon the LM5158 SEPIC Controller Design tool. This divider results in a turn on of approx 6.2V, while the turn off is at 5.6V.
What am I missing?
Hello Raymond,
Thanks for reaching out to us via e2e.
The UVLO input has a hysteresis of 50 mV between rising and falling edge, to avoid oscillations.
The quick start calculator is making use of that and offers you a calculation to tweak the hysteresis on the input voltage a little bit,
dependent on the resistor values.
To make this clear: This is NOT meant to give you full freedom for a big delta between the on and off voltages.
Your calculation somehow went wrong.
When I enter 2.24V and 6V, I get 712K and 237K as a result.
So I end up with an overall resistance which is close to 1 Megaohm.
I am pretty sure that the system would no longer work stable, when you go above maybe 200K or 300K.
And this is what I meant to say earlier on.
This concept with the small hysteresis is meant to avoid oscillations, not to allow for such a big delta between on and off voltages.
Best regards,
Harry
