In the datasheet, it says that the equation to adjust Vout is:

Vout = 1.25 (1 + R2/R1)

In my case, R1 is 220R, and I calculated R2 for three voltage values:

10V - 1.5K
13.8V - 2.2K
20V - 3.3K

Is it OK to ignore Iadj*R2 term (as they mention that Iadj is minimized)
and to only have a 5K potentiometer as R2 ?

Lucian

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Lucian
Sent: 15 mai 2004 09:27
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] Cascading Regulators

I put it next to a heatsink, but not tightly coupled, as it is isolated.
I hope it will not get too hot. My mistake in designing the circuit and
connecting the LM338 and LM7805 on the same heatsink.

Now my problem is solved and the circuit is working nice, apart from the
13.8V problem: I can't get 13.8V out of the LM338 regulator. The adjust
resistors are as follows: 220R between ADJ and VOUT and 2.2K plus a 5K
potentiometer between ADJ and GND. Am I mistaking the values of the
resistors ? If so, what would be some other better values ? (I have smd
resistors and I don't know how could I change them, but to know for the
future). Could the potentiometer be too bad ? As it jumps suddenly from
13.3 to 14.6+.

Lucian
-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Russell McMahon
Sent: 15 mai 2004 07:41
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] Cascading Regulators

> ICs. Anyway, th maximum current is around 500mA through the 7805.

(13.8 - 5 ) x 500 mA =~4.5w
Not vast but heatsink accordingly.

        RM

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