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From: acctforjunk on 23 Mar 2006 19:37
I'm using the LM2678-ADJ and have the output set to 4.4V. This is a
somewhat arbitrary value during my course of testing a circuit
including this regulator. I'm connecting a litte more than a 2ohm
resistive load across the output, so it should draw less than 2.5 amps;
well within the regulators 5 amp rating. I'm using a 100uH toroid
inductor, rated for >8 amps, a 1000uF cap at the input, and 470uF at
the output. Voltage is set with a 1K and 2.7K resistance.


Here's the problem: when I raise the input voltage to more than about
17V, the regulator's output starts to reduce. At 25V input, the output
drops to about 3V. The output is still pretty clean. This seems like
a feature, but it's a bad one in my design, and I can't find anything
in the datasheet regarding it.

Any ideas?

From: yomismo on 24 Mar 2006 04:45
May be, that 17V is the point of change from continuous to
discontinuous mode.

Try with smaller values in the inductor, for example 33uH.

From: acctforjunk on 24 Mar 2006 06:55

yomismo wrote:
> May be, that 17V is the point of change from continuous to
> discontinuous mode.
>
> Try with smaller values in the inductor, for example 33uH.

I finally figured it out. I needed more filtering on Vin. I put a
0.47 uF right across the power pins.

From: Phil Allison on 24 Mar 2006 07:53

<acctforjunk(a)yahoo.com>

> I finally figured it out. I needed more filtering on Vin. I put a
> 0.47 uF right across the power pins.
>


** A 0.47 uF what ???

The data sheet showed 3 x 15 uF electros.

Got them too ?





......... Phil


From: acctforjunk on 24 Mar 2006 19:57
Read my original post: "a 1000uF cap at the input".

Switchers Made Simple 6.24 specifies a 1000uF cap for nearly any
voltage/current combination, and this will eventually go into a
variable power supply. The 0.47uF is just for "up close and personal"
bypassing.


Phil Allison wrote:
> <acctforjunk(a)yahoo.com>
>
> > I finally figured it out. I needed more filtering on Vin. I put a
> > 0.47 uF right across the power pins.
> >
>
>
> ** A 0.47 uF what ???
>
> The data sheet showed 3 x 15 uF electros.
>
> Got them too ?
>
>
>
>
>
> ........ Phil

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