### Author Topic: Does the LM334 reference current source have a defined Rset range?  (Read 304 times)

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#### zdelarosa00

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##### Does the LM334 reference current source have a defined Rset range?
« on: March 13, 2019, 09:37:24 am »
Hello all!..

I was looking in some videos for practical applications, and the TI datasheet, but there doesn't seem to have an specified resistor range.

What it is defined is the operating voltage and the current limit of 10mA. So, if 40v is the max supply and 10mA the max set current, around 4kOhm?...

That is actually my question. Because there is many many appication circuits and not all of them are exactly just references or plain current sources. So I was planning to make a component list and I want to know if I need a consideration for the set resistor value.

Have a good day people, and thanks in advance.
« Last Edit: March 13, 2019, 09:40:12 am by zdelarosa00 »
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#### newbrain

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##### Re: Does the LM334 reference current source have a defined Rset range?
« Reply #1 on: March 13, 2019, 11:01:34 am »
Rset is independent of the supply voltage.

Its value is given (in normal conditions) by Eq 3 on page 6 of the datasheet.

E.g. at 25° C == 298,15 K, for a current of 10mA, we have:

10 mA = 227 uV/K * 298.15 K / Rset

Rset = 227E-6 * 298.15 / 10E-3

Rset ~= 6.8

With Rset = 4000 the current will be ~ 17 µA.

Of course, one must be careful about power dissipation: with 40 V across the device and 10 mA through it, we have 400 mW.
In a TO-92, at 180 K/W and 25° C ambient temperature, the "junction" will be at 180 * 0.4 + 25 = 97° C, exceeding the maximum operating temperature of an LM334 by 27 degrees (but what about Note 4?).
« Last Edit: March 13, 2019, 03:21:41 pm by newbrain »
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#### David Hess

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##### Re: Does the LM334 reference current source have a defined Rset range?
« Reply #2 on: March 13, 2019, 02:26:32 pm »
At the low end, 1 microamp requires a 68 kilohm set resistance.

Smf