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From: Chris Lee on 2 Oct 2008 02:23
I use two dc/dc regulator to generate 12V and 5V output from 40V
input.

40V --- LM5576 --- 12V

40V --- LM5576 --- 5V

Since LM5576 is too expensive, I wonder if I can use a cheaper dc/dc
converter in cascade with LM5576 to generate 5V from 12V as the
following?
I can not use a LDO for 12V to 5V because the loading is about 1.5A,
the board will get too hot.

40V --- LM5576 --- 12V --- AT1796 --- 5V


From: John Popelish on 2 Oct 2008 12:45
Chris Lee wrote:
> I use two dc/dc regulator to generate 12V and 5V output from 40V
> input.
>
> 40V --- LM5576 --- 12V
>
> 40V --- LM5576 --- 5V
>
> Since LM5576 is too expensive, I wonder if I can use a cheaper dc/dc
> converter in cascade with LM5576 to generate 5V from 12V as the
> following?
> I can not use a LDO for 12V to 5V because the loading is about 1.5A,
> the board will get too hot.
>
> 40V --- LM5576 --- 12V --- AT1796 --- 5V

Are you certain that your 40 to 12 converter will handle the
additional current? If the 12 to 5 volt converter is 85%
efficient, it will add about (5*1.5)/(0.8*12)=0.78 amps load
, average, to the 12 volt output.

This one is about a dollar cheaper:
http://search.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=LM2670S-5.0-ND



--
Regards,

John Popelish
From: John Popelish on 2 Oct 2008 13:03
Here are a couple more possibilities:
http://search.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=FAN2106MPXCT-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=FAN2103EMPXCT-ND

--
Regards,

John Popelish
From: jpopelish on 2 Oct 2008 21:32
http://search.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=296-19671-5-ND

--
Regards,

John Popelish
From: David L. Jones on 3 Oct 2008 02:06
On Oct 2, 4:23 pm, Chris Lee <ChungHua...(a)gmail.com> wrote:
> I use two dc/dc regulator to generate 12V and 5V output from 40V
> input.
>
> 40V --- LM5576 --- 12V
>
> 40V --- LM5576 --- 5V
>
> Since LM5576 is too expensive, I wonder if I can use a cheaper dc/dc
> converter in cascade with LM5576 to generate 5V from 12V as the
> following?
> I can not use a LDO for 12V to 5V because the loading is about 1.5A,
> the board will get too hot.
>
> 40V --- LM5576 --- 12V --- AT1796 --- 5V

Yes, you can do that, provided your LM5576 can handle the new total
load including inefficiencies.

There are squillions of regulators, so there are most likely cheaper
options available.
Pick some parts you like and use http://www.findchips.com/ to compare
prices and availability.

Dave.
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