Question about Iadj in LM317

[Ethan]

I've read over the app sheet for the LM317, and I'm a bit perplexed about the Iadj. terminal. Is that to adjust output current? Me confooosed. Or more specifically I'm confused about how to determine the Iadj. to plug into the Vout equation.

http://cache.national.com/ds/LM/LM117.pdf

 

[alk509]

[quote author="Ethan"]the Iadj. terminal. Is that to adjust output current?[/quote]

There actually isn't a Iadj terminal... Iadj refers to the current that flows out of the Adj terminal.

Peace,
Al.

 

[alk509]

[quote author="Ethan"]how to determine the Iadj. to plug into the Vout equation.[/quote]

Iadj is a constant. It should be around 100 micro amps or so - check the datasheet.

Peace,
Al.

EDIT: OK, from the datasheet you posted:

Adjustment Pin Current: 50uA (Typical), 100uA (Max)

Peace,
Al.

 

[PRR]

> how to determine the Iadj. to plug into the Vout equation.

It varies.

But what you do is set I1 be much-much more than Iadj. Anyway you need 4mA of load on the 317 to keep it alive, and the adjustment network is a handy place to do it. They suggest that R1 be 240Ω, which gives I1 about 5mA. Then the typical 0.050mA Iadj adds a 1% error, negligible, ignore it.

So set R1=240Ω and find R2 from Vout - 1.25V*(R1+R2)/R1. Or R2= (Vout-1.25V)/5.2mA. The error due to Iadj vanishes in other tolerances.

In micropower work you might not want 5mA in the adjust network and might pretend that the load will always be present to meet the 317's minimum load current. Then you might have to allow for Iadj. You don't want to go there.

 

[Ethan]

Beautiful Explanation.
I read through the whole app sheet which for the most part was very clear and thorough, but the adj terminal wasn't explaned very well (for me), but that clears it up. Thanks!

 

[PRR]

From http://www.national.com/kbase/category/Power.html#157

How do I calculate the values needed for the feedback resistors of adjustable linear regulators, such as the LM317?

Two resistors are used to set the value of the output voltage for adjustable regulators.

For adjustable standard linear regulators, such as LM317 and LM337, R1 is the resistor between Vout and ADJ, while R2 goes from ADJ to ground.

The values of the resistors are determined from this formula: Vout = Vref (1 + R2 / R1). R1 is generally specified on the datasheet, and you must solve for R2.

A more precise form of the equation allows for adjust pin bias current: Vout = Vref (1 + R2 / R1) + (Iadj * R2).

Vref usually is 1.25V for linear (LM317/337 type) regulators... For linear regulators, R1 is usually 240Ω. Iadj is typically 50µA for the LM317/337. See Application Note AN-181, "3-Terminal Regulator is Adjustable", for more information about the LM317.

From AN-181 (which is really at http://www.national.com/an/AN/AN-181.pdf )

a divider R1 and R2 is connected from the output to ground as is shown in Figure 2. The 1.2V reference across resistor R1 forces 5 mA of current to flow. This 5 mA then flows through R2, increasing the voltage at the adjustment terminal and therefore the output voltage. The output voltage is given by:

Vout = 1.2V * (1 + R2/R1) + (50uA * R2)

The 50 µA biasing current is small compared to 5 mA and causes only a small error in actual output voltages.