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Buck converter thing

I had a need for a circuit that can convert 12-24 volts to 5 volts for logic at a few hundred mA, and I wanted to build it with parts that I have on hand. Some of the hysteretic buck converter circuits I've seen use a supply voltage and temperature stable reference, like an LM336, to regulate the output voltage. Unfortunately, I don't have any of those, but I do have some funky-junky 4.7 volt zeners!

I found that using them with a few extra components and the right start-up circuit, one can hijack the output of the 5V post regulator to use as the reference once the circuit is running. Hopefully the output of the regulator will be more stable with regards to temperature and supply voltage changes than just using the zener.

Here's what I came up with:

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Reply to
Bitrex
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On a sunny day (Wed, 20 Apr 2011 06:40:33 -0400) it happened Bitrex wrote in :

Looks complicated, this is what I use for my 3.3V embedded:

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(copy from some page of the LM317 datsheet). I use a smaller L, 120 uH IIRC, would have to look it up.

Reply to
Jan Panteltje

That's a neat way to use the internal 1.25 volt reference of the LM317 as the reference - I didn't think of that. How's the ripple on the output? I'll give that one a try.

Reply to
Bitrex

On a sunny day (Wed, 20 Apr 2011 07:51:08 -0400) it happened Bitrex wrote in :

Not much, do not remember the exact value, I have 150 uF on the output in my case:

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I see now I used 230 uH. It is only digital logic so the ripple is not that critical. I found that LM317s from different manufacturers behave very differently. In once case I had to leave out that feedback capacitor, you may have to experiment with that value. That does something to the frequency and then determines the value of L. I added some turns to some old filter core to get the 230 uH:

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It is reliable, been working OK for about a year 24/7.

Reply to
Jan Panteltje

The other neat thing about that circuit is that when Vin pulls down to the output voltage, as it does to turn on Q1 when the comparator trips and the internal pass transistor turns on, the voltage at Vin still has to power the the internal comparator. Looks like it will still operate down to a pretty low voltage.

Reply to
Bitrex
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On 4/20/2011 8:18 AM, Jan Panteltje wrote:

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I can't seem to get it to work in simulation, at least with the LM317 model that I'm using. If anyone else wants to try model here's the circuit and model, I'll attempt to attach the component symbol too:

Version 4 SHEET 1 880 680 WIRE -176 -48 -320 -48 WIRE -48 -48 -176 -48 WIRE 352 -48 48 -48 WIRE 384 -48 352 -48 WIRE 528 -48 464 -48 WIRE 704 -48 528 -48 WIRE 704 80 704 -48 WIRE -176 112 -176 -48 WIRE -96 112 -176 112 WIRE -80 112 -96 112 WIRE 0 112 0 16 WIRE 48 112 0 112 WIRE 224 112 208 112 WIRE 320 112 304 112 WIRE 528 112 528 -48 WIRE 528 112 320 112 WIRE 528 128 528 112 WIRE -320 144 -320 -48 WIRE -96 224 -96 112 WIRE 128 224 128 192 WIRE 128 224 48 224 WIRE 192 224 128 224 WIRE 320 224 320 112 WIRE 320 224 272 224 WIRE 48 304 48 288 WIRE 128 304 48 304 WIRE 192 304 128 304 WIRE 352 304 352 -48 WIRE 352 304 272 304 WIRE 352 320 352 304 WIRE 128 336 128 304 WIRE 192 352 192 304 WIRE 208 352 192 352 WIRE 272 352 272 304 WIRE -320 416 -320 224 WIRE -96 416 -96 288 WIRE -96 416 -320 416 WIRE 128 416 -96 416 WIRE 352 416 352 384 WIRE 352 416 128 416 WIRE 528 416 528 192 WIRE 528 416 352 416 WIRE 704 416 704 160 WIRE 704 416 528 416 SYMBOL lm317 128 128 R0 SYMATTR InstName U1 SYMBOL polcap -112 224 R0 WINDOW 3 24 64 Left 0 SYMATTR Value 47µ SYMATTR InstName C1 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=35 Irms=170m Rser=1 Lser=0 mfg="Nichicon" pn="UPG1V470MPH" type="Al electrolytic" SYMBOL res 112 320 R0 SYMATTR InstName R1 SYMATTR Value 100 SYMBOL res 112 208 R0 SYMATTR InstName R2 SYMATTR Value 4.7k SYMBOL cap 32 224 R0 SYMATTR InstName C2 SYMATTR Value 0.01µF SYMBOL res 288 288 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 -22 -2 VTop 0 SYMATTR InstName R3 SYMATTR Value 15k SYMBOL cap 272 336 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 300p SYMBOL schottky 368 384 R180 WINDOW 0 24 72 Left 0 WINDOW 3 -99 34 Left 0 SYMATTR InstName D1 SYMATTR Value 1N5819 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 320 96 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 55 VTop 0 SYMATTR InstName R4 SYMATTR Value 0.25 SYMBOL res 288 208 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 240 SYMBOL ind 480 -64 R90 WINDOW 0 5 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName L1 SYMATTR Value 220µH SYMBOL pnp -48 16 R270 SYMATTR InstName Q1 SYMATTR Value KSA473 SYMBOL res 16 96 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 22 SYMBOL current 704 80 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value 0.2 SYMBOL voltage -320 128 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 24 SYMBOL polcap 512 128 R0 WINDOW 3 24 64 Left 0 SYMATTR Value 100µ SYMATTR InstName C4 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=10 Irms=170m Rser=1 Lser=0 mfg="Nichicon" pn="UPG1A101MPH" type="Al electrolytic" TEXT -352 -208 Left 0 !.tran 0.1 startup

*LM317 Adjustable Voltage Regulator *Adjustable Voltage Regulator pkg:TO-220 3,1,2 *Connections:
  • Input
  • | Adjust
  • | | Output
  • | | | .SUBCKT LM317 1 2 3 J1 1 3 4 JN Q2 5 5 6 QPL .1 Q3 5 8 9 QNL .2 Q4 8 5 7 QPL .1 Q5 81 8 3 QNL .2 Q6 3 81 10 QPL .2 Q7 12 81 13 QNL .2 Q8 10 5 11 QPL .2 Q9 14 12 10 QPL .2 Q10 16 5 17 QPL .2 Q11 16 14 15 QNL .2 Q12 3 20 16 QPL .2 Q13 1 19 20 QNL .2 Q14 19 5 18 QPL .2 Q15 3 21 19 QPL .2 Q16 21 22 16 QPL .2 Q17 21 3 24 QNL .2 Q18 22 22 16 QPL .2 Q19 22 3 241 QNL 2 Q20 3 25 16 QPL .2 Q21 25 26 3 QNL .2 Q22A 35 35 1 QPL 2 Q22B 16 35 1 QPL 2 Q23 35 16 30 QNL 2 Q24A 27 40 29 QNL .2 Q24B 27 40 28 QNL .2 Q25 1 31 41 QNL 5 Q26 1 41 32 QNL 50 D1 3 4 DZ D2 33 1 DZ D3 29 34 DZ R1 1 6 310 R2 1 7 310 R3 1 11 190 R4 1 17 82 R5 1 18 5.6K R6 4 8 100K R7 8 81 130 R8 10 12 12.4K R9 9 3 180 R10 13 3 4.1K R11 14 3 5.8K R12 15 3 72 R13 20 3 5.1K R14 2 24 12K R15 24 241 2.4K R16 16 25 6.7K R17 16 40 12K R18 30 41 130 R19 16 31 370 R20 26 27 13K R21 27 40 400 R22 3 41 160 R23 33 34 18K R24 28 29 160 R25 28 32 3 R26 32 3 .1 C1 21 3 30PF C2 21 2 30PF C3 25 26 5PF CBS1 5 3 2PF CBS2 35 3 1PF CBS3 22 3 1PF .MODEL JN NJF(BETA=1E-4 VTO=-7) .MODEL DZ D(BV=6.3) .MODEL QNL NPN(EG=1.22 BF=80 RB=100 CCS=1.5PF TF=.3NS TR=6NS CJE=2PF
  • CJC=1PF VAF=100) .MODEL QPL PNP(BF=40 RB=20 TF=.6NS TR=10NS CJE=1.5PF CJC=1PF VAF=50) .ENDS LM317
Reply to
Bitrex
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On 4/20/2011 8:23 AM, Bitrex wrote:

I can't seem to get it to work in simulation. :( If anyone else wants to try, here's the circuit and the model and symbol for the LM317 I'm using. I'll try attaching the files too.

Version 4 SHEET 1 880 680 WIRE -176 -48 -320 -48 WIRE -48 -48 -176 -48 WIRE 352 -48 48 -48 WIRE 384 -48 352 -48 WIRE 528 -48 464 -48 WIRE 704 -48 528 -48 WIRE 704 80 704 -48 WIRE -176 112 -176 -48 WIRE -96 112 -176 112 WIRE -80 112 -96 112 WIRE 0 112 0 16 WIRE 48 112 0 112 WIRE 224 112 208 112 WIRE 320 112 304 112 WIRE 528 112 528 -48 WIRE 528 112 320 112 WIRE 528 128 528 112 WIRE -320 144 -320 -48 WIRE -96 224 -96 112 WIRE 128 224 128 192 WIRE 128 224 48 224 WIRE 192 224 128 224 WIRE 320 224 320 112 WIRE 320 224 272 224 WIRE 48 304 48 288 WIRE 128 304 48 304 WIRE 192 304 128 304 WIRE 352 304 352 -48 WIRE 352 304 272 304 WIRE 352 320 352 304 WIRE 128 336 128 304 WIRE 192 352 192 304 WIRE 208 352 192 352 WIRE 272 352 272 304 WIRE -320 416 -320 224 WIRE -96 416 -96 288 WIRE -96 416 -320 416 WIRE 128 416 -96 416 WIRE 352 416 352 384 WIRE 352 416 128 416 WIRE 528 416 528 192 WIRE 528 416 352 416 WIRE 704 416 704 160 WIRE 704 416 528 416 SYMBOL lm317 128 128 R0 SYMATTR InstName U1 SYMBOL polcap -112 224 R0 WINDOW 3 24 64 Left 0 SYMATTR Value 47µ SYMATTR InstName C1 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=35 Irms=170m Rser=1 Lser=0 mfg="Nichicon" pn="UPG1V470MPH" type="Al electrolytic" SYMBOL res 112 320 R0 SYMATTR InstName R1 SYMATTR Value 100 SYMBOL res 112 208 R0 SYMATTR InstName R2 SYMATTR Value 4.7k SYMBOL cap 32 224 R0 SYMATTR InstName C2 SYMATTR Value 0.01µF SYMBOL res 288 288 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 -22 -2 VTop 0 SYMATTR InstName R3 SYMATTR Value 15k SYMBOL cap 272 336 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 300p SYMBOL schottky 368 384 R180 WINDOW 0 24 72 Left 0 WINDOW 3 -99 34 Left 0 SYMATTR InstName D1 SYMATTR Value 1N5819 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 320 96 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 55 VTop 0 SYMATTR InstName R4 SYMATTR Value 0.25 SYMBOL res 288 208 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 240 SYMBOL ind 480 -64 R90 WINDOW 0 5 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName L1 SYMATTR Value 220µH SYMBOL pnp -48 16 R270 SYMATTR InstName Q1 SYMATTR Value KSA473 SYMBOL res 16 96 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 22 SYMBOL current 704 80 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value 0.2 SYMBOL voltage -320 128 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 24 SYMBOL polcap 512 128 R0 WINDOW 3 24 64 Left 0 SYMATTR Value 100µ SYMATTR InstName C4 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=10 Irms=170m Rser=1 Lser=0 mfg="Nichicon" pn="UPG1A101MPH" type="Al electrolytic" TEXT -352 -208 Left 0 !.tran 0.1 startup

*LM317 Adjustable Voltage Regulator *Adjustable Voltage Regulator pkg:TO-220 3,1,2 *Connections:
  • Input
  • | Adjust
  • | | Output
  • | | | .SUBCKT LM317 1 2 3 J1 1 3 4 JN Q2 5 5 6 QPL .1 Q3 5 8 9 QNL .2 Q4 8 5 7 QPL .1 Q5 81 8 3 QNL .2 Q6 3 81 10 QPL .2 Q7 12 81 13 QNL .2 Q8 10 5 11 QPL .2 Q9 14 12 10 QPL .2 Q10 16 5 17 QPL .2 Q11 16 14 15 QNL .2 Q12 3 20 16 QPL .2 Q13 1 19 20 QNL .2 Q14 19 5 18 QPL .2 Q15 3 21 19 QPL .2 Q16 21 22 16 QPL .2 Q17 21 3 24 QNL .2 Q18 22 22 16 QPL .2 Q19 22 3 241 QNL 2 Q20 3 25 16 QPL .2 Q21 25 26 3 QNL .2 Q22A 35 35 1 QPL 2 Q22B 16 35 1 QPL 2 Q23 35 16 30 QNL 2 Q24A 27 40 29 QNL .2 Q24B 27 40 28 QNL .2 Q25 1 31 41 QNL 5 Q26 1 41 32 QNL 50 D1 3 4 DZ D2 33 1 DZ D3 29 34 DZ R1 1 6 310 R2 1 7 310 R3 1 11 190 R4 1 17 82 R5 1 18 5.6K R6 4 8 100K R7 8 81 130 R8 10 12 12.4K R9 9 3 180 R10 13 3 4.1K R11 14 3 5.8K R12 15 3 72 R13 20 3 5.1K R14 2 24 12K R15 24 241 2.4K R16 16 25 6.7K R17 16 40 12K R18 30 41 130 R19 16 31 370 R20 26 27 13K R21 27 40 400 R22 3 41 160 R23 33 34 18K R24 28 29 160 R25 28 32 3 R26 32 3 .1 C1 21 3 30PF C2 21 2 30PF C3 25 26 5PF CBS1 5 3 2PF CBS2 35 3 1PF CBS3 22 3 1PF .MODEL JN NJF(BETA=1E-4 VTO=-7) .MODEL DZ D(BV=6.3) .MODEL QNL NPN(EG=1.22 BF=80 RB=100 CCS=1.5PF TF=.3NS TR=6NS CJE=2PF
  • CJC=1PF VAF=100) .MODEL QPL PNP(BF=40 RB=20 TF=.6NS TR=10NS CJE=1.5PF CJC=1PF VAF=50) .ENDS LM317

Version 4 SymbolType BLOCK RECTANGLE Normal 80 64 -80 -48 TEXT 0 0 Top 0 LM317 SYMATTR Prefix X SYMATTR SpiceModel LM317 SYMATTR ModelFile LM317.lib PIN -80 -16 LEFT 8 PINATTR PinName IN PINATTR SpiceOrder 1 PIN 80 -16 RIGHT 8 PINATTR PinName OUT PINATTR SpiceOrder 3 PIN 0 64 BOTTOM 2 PINATTR PinName ADJ PINATTR SpiceOrder 2

Reply to
Bitrex

where does the current from Q1 collector go ?

why pullup on U3 output ?

Where is cap to filter switching currents of L1 ?

What does R4 do ?

why use comparitors instead of op amps ?

Vcc input - 1.4 volts is across R8, what is current sinking of U1 ?

Reply to
anospam

It keeps C5 charged up to the 5 volts from the output I'm using as the reference. C5 does slowly discharge when Q3 switches off and the voltage across D1 goes to ~ -0.6 volts.

The LM*39/93 comparator series have open collector outputs, they need a pullup to define the HIGH output level.

C2

R4 and R3 set the hysteresis for the hysteretic converter, just like a Schmitt trigger. Because the Darlington inverts, the inputs to the comparator are actually reversed, the - input is + and vice versa.

An op amp as inexpensive as a dual comparator like the LM393 probably wouldn't function in the circuit as well because of its more limited slew rate.

That resistor should probably be more like 22k so U1 sinks about a mA when the Darlington turns on. Too large and it will slow down the rise and fall times of the square wave.

Reply to
Bitrex

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