High current low voltage regulator

Thread Starter

tinker123

Joined Mar 6, 2020
15
I have a project (hobby not commercial) where I am trying to replace a pair of 18650 cells in series with a 12v power input socket and a voltage regulator circuit.

Requirements:
- Input: 12v (probably a laptop inline power supply)
- Load: black box heating circuit that requires 8.4 V and 6 A (would prefer to design for 8 A for safety) input
- Load pattern: Device pulses on for 30 seconds at a time, running no more than 2 minutes out of 5 (mentioning this because it leaves time for heat dissipation)
- Build Space: Entire regulation circuit with power socket needs to fit in battery cell space, 18mm by 65mm long with near zero air flow.
- Cost: under $15 (there is flexibility here)

I expected to find an existing regulator, similar to an LM338, that would support this but I can't find one on Digikey that supports > 5A. Can you point me to a single chip regulator that would work in this tight space? Would greatly prefer through hole for ease of building. I am considering whether I can push a single LM338 this hard. If it reliably self protects and only shuts off occasionally I can live with that.

I considered MOSFET options but I worry they will generate too much heat in this tight space.

I've seen circuits for paralleling two LM338's with 0.3 Ohm series output resistors and a current regulating feedback opamp to balance. Again I'm worried about the heat and inefficiency with pumping 6 amps through those resistors.

Any suggestions greatly appreciated. There has to be a chip out there I haven't found...
 

ElectricSpidey

Joined Dec 2, 2017
1,258
You are going to have to dissipate 21.6 watts with any linear regulation.

You might find a switch mode regulator that can do 6 amps...but the size and price restrictions are going to be difficult.

If the load is a constant resistive one, you could skip the regulator and just use a power resistor, with a 18 x 65mm heatsink.

Good luck...with those requirements.
 

Thread Starter

tinker123

Joined Mar 6, 2020
15
I may have found what I'm looking for searching on Mouser.com. Does anyone know anything about the LT1270 , it's a switching regulator in a 5 pin TO package. If anyone is bored and has a moment to look over that spec please let me know if you see any red flags regarding my intended use.

LT1270BuckRefCircuit.PNG

This circuit pattern is a little different than I am used to. I guess the LT1431CZ 2.5 V voltage reference forces the center of the voltage divider to 2.5 and does what exactly? Stabilizes the output maybe? Why wouldn't it already be fairly stable? All I need to do is increase that 9.1K resistor to 34K to get my 8.5 V output, yes?

Thanks all for your patience, I was thoroughly educated in this stuff, about 35 years ago...
 

Thread Starter

tinker123

Joined Mar 6, 2020
15
I may have found what I'm looking for
Oops, no this isn't it. I missed that this is a switching controller and the main power capacitor and inductor are external and fairly large. lt looks like only a linear regulator will do the job. Sorry for the spam...
 
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