lm338T

P

power453

New Member

• Mar 21, 2007

• #1

Hey every body. I need some help. I want to put some lm338's in a circuit as a fixed high current voltage regulator. I need to know how to calcuate the values. The national datasheet gives the formula except for one thing. What is Iadj all about? I am guessing this is the maximum current you intend to use with it. But i'm not sure. Some help would be nice.
Thankyou.
R. Ellis

 

Sceadwian

Banned

• Mar 21, 2007

• #2

Iadj is the input back into the regulator from the feedback network on the output that tells the regulator what the output voltage is so it can actually regulate the voltage.

 

audioguru

Well-Known Member

Most Helpful Member

• Mar 21, 2007

• #3

Just connect a 120 ohm resistor from the output to the ADJ pin. It will have 1.25V across it so its current is 1.25V/120 ohms= 10.42mA. The resistor from the ADJ pin to ground is calculated with this current in it and the voltage created by the current is added to the 1.25V to result in the output voltage.

 

Gayan Soyza

Active Member

• Mar 22, 2007

• #4

For constant current you can use this circuit
**broken link removed**
ex:

Code:

1.2V/0.24R=5A (1.2V is the voltage between ADJ & output)

Change the resistor value with suitable wattage for different amphere range.

 

Hero999

Banned

• Mar 22, 2007

• #5

LM338Ts are a wasteful way for doing constant current regulation, you can loose up to 4.25V and at 5A thats 21.25W!

For high currents it's better to use a switching regulator.

 

P

power453

New Member

• Mar 22, 2007

• #6

I don't want constant current. I need constant voltage. Sorry for the confusion.

 

Roff

Well-Known Member

• Mar 22, 2007

• #7

The value for Iadj is specified in the datasheet. In the National datasheet, it is typically 45uA, with a maximum value of 100uA. It is the current coming out of the Adj pin. If you are passing 10mA through the voltage setting resistors, you can generally ignore Iadj, as your resistor tolerances and the ref. voltage tolerance of the 338 will overshadow it. If you are using a pot for adjustment, you can certainly ignore it.

 

Hero999

Banned

• Mar 22, 2007

• #8

What's the input voltage?

Have you considered using a switching regulator like the MAX649?

 

P

power453

New Member

• Mar 22, 2007

• #9

INput will be around 25 voltsdc. I havn't considerd switching types. They are usually too complex for just a simple regulation requirement on a line powered device.

 

Hero999

Banned

• Mar 23, 2007

• #10

What's the output voltage?

Switching regulators aren't complicated at all, most moderd IC only require a few external components and it can work out cheaper because you don't need a bulky heatsink.