Correct Use of LM337

[cmbeelby]

Hello,

If you have a negative voltage like say -20V and you want to get it down to -17V (or technically this is up since -17 is greater than -20) would you use the LM337 in this case?

The current would be about 10mA - 13mA in my project with an absolute maximum of 20mA if that matters.

I found a **broken link removed** and it seems that if I use values of 160Ω for R1 and 2KΩ for R2 then I will get about -17V output if I am understanding it correctly.

I already ordered some resistors. For the 2K resistor I got one that is rated for 1-watt of power since that same page said it would need to handle 123 mW and I figured I should be safe. For R1 the calculator only says it will need to handle 9.7 mW so I figured the standard 1/4 watt resistor should be fine. Please let me know if what I am saying is correct or if I am making wrong assumptions. Thanks.

 

[birdman0_o]

If you used the calculator and it gave the right values, then what you are saying seems right to me!

 

[audioguru]

R1 should be 120 ohms for an LM337 or the output voltage might rise without a load. It can be as high as 240 ohms for a more expensive LM137. Then R2's value should be re-calculated.

 

[cmbeelby]

R1 should be 120 ohms for an LM337 or the output voltage might rise without a load. It can be as high as 240 ohms for a more expensive LM137. Then R2's value should be re-calculated.

So using 120Ω for R1 gives about 1500Ω for R2 with similar power requirements (1/4 watt for R1 and 1 watt for R2 to be very safe). However I did not find anything in the documentation saying why I should use 120 over any other value. I'm just trying to understand how you came up with that value.

The chip I am looking at is here and the document I am refering to is here.

 

[audioguru]

The max operating current of the LM317 and LM337 is 10mA which goes to the output. The datasheet calls it the Minimum Load Current. The max operating current for the more expensive LM117 and LM137 is only 5mA. The resistor from the output to the ADJ terminal conducts this current through the pot to ground when there is no load.

The voltage across R1 is 1.25V. If it is 120 ohms then the current is 1.25V/120 ohms= 10.4mA and the output voltage will be regulated. But if R1 is 220 ohms then the current is only 5.7mA so the 1.25V reference will be increased to 220 ohms x 10mA= 2.2V which causes the output voltage to rise.

 

[Speakerguy]

Audioguru is correct, you should use a 120 (or 121) ohm resistor with any 337 type regulator. The Fairchild data sheet doesn't mention it, but that's because it's a copy of the National Semiconductor LM337 and generic parts don't often have very thorough data sheets. You have to look at the original mfg for a thorough data sheet, which will most definitely mention using a 120ohm resistor.

The Fairchild data sheet does give all its specifications at Iout >10ma. This is a roundabout way of saying you need to use a ~120 ohm resistor on the ADJ terminal because the internal voltage reference is ~1.25V or so.

 

[cmbeelby]

From the specs

So yes in the document that comes with the chip it does mention that the "reference voltage" is -1.25V when the conditions are such that the I_o is between 10mA and 1.5A.

I also see a row called "Minimum load current to maintain regulation" which itself has a maximum value of 10mA, with a "typical" value of 2.5mA. So is the "maximum" value of the "minimum load" a way of saying that it must be at least 10mA ? I noticed in the calculator that I referenced when I put in the values that audioguru suggested it does calculate a current over just slightly over 10mA. So is this a rule of thumb when calculating the values that I need to make sure I am getting at least 10mA of current? Can I go over the 10mA (up to 1.5A?) and still be safe or do I want to stick close to 10mA?

Sadly I already ordered the resistors in the wrong values, but that's okay. I'll just keep them around and order some more. Resistors are so much cheaper on digi-key than at Radioshack that even with shipping if I buy at least 10 resistors I come out ahead.

 

[audioguru]

If you use 120 ohms for R1 then every LM317 and LM337 will have good voltage regulation without a load.
If you use 220 ohms for R1 then some LM317 and LM337 ICs will have their voltage rise without a load.