# Startup voltage help: How can I change the input voltage to this IC when starting the circuit?

I am currently trying to find a solution to switching the supply voltage for U12 (PN: UC3842BVD1R2G) in the included schematic. What is special here is that U12 will draw 22mA no matter the input voltage I tested at, I did a sweep from 13v to 30v and it always only pulled 23mA. The issue here, is that even though it can be supplied with 30v, running it off the usual 24v causes the chip to run at 170 F, which in my case is a problem, and is near the high end of the recommended temps. Running the chip at 15v however, causes it to run at about 120 F, much better. The other issue with just trying to connect a 15v regulator right away is that the chip needs over 16v to start the circuit, then can be brought back down to 12-15v and run off of that.

I am attempting to start u12 on the input voltage, then when the regulator starts up, to use that to turn off Q1, changing the input from the 24v line to the 15v line. However, I don't think that would work, and was wondering if there is a normally closed held open kind of device that could do something like this? I also don't have a lot of space on this board for some type of relay either, or that would have been first choice.

Any help appreciated.

• You can use a thermistor (Negative temp coefficient) and a normal resistor and create a voltage divider that divides rectifier 24V. After certain time, the resistance of the thermistor decreases and its voltage drop decreases. So you start off with 17V then drop below 15V. Commented Aug 13, 2020 at 17:02
• Can you use the larger package with the split Vcc and Vc? then you could run them at seperate voltages and use a circuit to give the 5V regulator 6V and dissipate the power outside of the part, then run Vc at whatever you want. Commented Aug 13, 2020 at 17:09
• Why not just use UC3843B, with lower UVLO threshold? Commented Aug 13, 2020 at 17:11
• @ThePhoton currently, this is a replacement for the 3843B due to availability Commented Aug 13, 2020 at 17:41
• Look at Fig 17 in the datasheet. You can use a weak pull-up to Vin diode-or'd to your 15V supply. Make sure to add a bypass caps near the Vcc pin. Commented Aug 13, 2020 at 17:55