Current source with LM234

[treez]

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:
https://www.ti.com/lit/ds/symlink/lm234.pdf

 

[erikl]

I'd try a single chip solution with R_SET = (12Ω || 100Ω) . Use the 100Ω Poti as trimmer for the required current.

 

[treez]

....sorry but pot adjustment is an extra production procedure that we dont want to pay for.

 

[erikl]

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

 

[godfreyl]

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.

 

[crutschow]

What current accuracy do you require?

 

[treez]

accuracy is 6mA +/2mA

- - - Updated - - -

accuracy is 6mA +/2mA

 

[crutschow]

accuracy is 6mA +/2mA

OK, that should be doable if you use a 1% resistor to set the current level.

 

[ZekeR]

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...