# Current source with LM234

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#### treez

##### Advanced Member level 5
Hello,

I wish to make a 6.5mA current source with LM234.

The ratio of I(bias) to I(set) isn't quoted in the datasheet for any more than 5mA, therefore, i must use two pieces of LM234 and parallel them. (3.25mA each)

This means using a resistor of 21.2 Ohms in each LM234 circuit.

1....Is it ok to parallel LM234's

2...The datasheet page 3 quotes the ratio of I(set) to I(bias) as 14 for Iset= 1mA to 5mA, ...However, the graph on page 5 of the datasheet clearly shows that this ratio varies widely over the range 1mA to 5mA.....so how come they have quoted a fixed figure of "14" on page 3 of the datasheet?

3.....Why does the datasheet indicate that LM234 can supply 10mA?....after all the ratio of I(bias) to I(set) isnt stated for any more than 5mA....so surely 5mA is the realistic maximum for LM234?

LM234 DATASHEET:
http://www.ti.com/lit/ds/symlink/lm234.pdf

#### erikl

##### Super Moderator
Staff member
I'd try a single chip solution with RSET = (12Ω || 100Ω) . Use the 100Ω Poti as trimmer for the required current.

treez

### treez

points: 2
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#### treez

##### Advanced Member level 5
....sorry but pot adjustment is an extra production procedure that we dont want to pay for.

#### erikl

##### Super Moderator
Staff member
... sorry but this wasn't clear from your original statement. I'd suggest to try a 1% tolerance resistor for RSET (from the E96 series, if you can afford it) and see if you can live with it. Probably RSET ≈ 10.7Ω .

treez

### treez

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#### godfreyl

##### Advanced Member level 5
Extrapolating the graph, it looks like the ratio will be about 11.5 @ 6.5mA, so the resistance needs to be about 10.7 Ohms. One could always measure a sample to be sure, and make a prototype.

treez

### treez

points: 2
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#### crutschow

##### Advanced Member level 5
What current accuracy do you require?

treez

### treez

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#### treez

##### Advanced Member level 5
accuracy is 6mA +/2mA

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accuracy is 6mA +/2mA

#### crutschow

##### Advanced Member level 5
accuracy is 6mA +/2mA
OK, that should be doable if you use a 1% resistor to set the current level.

#### ZekeR

##### Full Member level 5
One thing you should keep in mind is that the current provided by the LM234 will be proportional to absolute temperature (PTAT). So if you program for 5mA at room temp (300K), then at 150°C (425K) it'll be 7.1mA and at -50°C (225K) it'll be 3.75mA. You can easily see this on the bench with a heat gun and freeze spray. This feature is useful for biasing BJT amplifiers, but not so helpful for some other things...

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