voltage regulator configuration

Hi,

I have a power requirement for +5V, +3.3V and +1.8V.

Studying available designs, I have seen both parallel and series
configuration.

1. Series

+5V DC input with voltage regulators connected in series to convert +5V to
+3.3V, and +3.3V to +1.8V

2. Parallel

+5V DC input to +3.3V output, and +5V DC input to +1.8V output.

In both cases, all three voltages are available.

What are the pros and cons?

Also, in the Olimex schematic (top right corner), I see a Schottky Barrier
being used (BAT54C),
http://www.olimex.com/dev/images/ARM/ATMEL/SAM9-L9260-REV-B-sch.gif

I believe the main purpose was to isolate the +5V, +3.3V and +1.8V power
rails. Is this really required?

Thanks,
Saritha.

On Jun 13, 4:08 pm, "kalyanamsaritha" <kalyanamsari...@gmail.com>
wrote:
> Hi,
>
> I have a power requirement for +5V, +3.3V and +1.8V.
>
> Studying available designs, I have seen both parallel and series
> configuration.
>
> 1. Series
>
> +5V DC input with voltage regulators connected in series to convert +5V to
> +3.3V, and +3.3V to +1.8V
>
> 2. Parallel
>
> +5V DC input to +3.3V output, and +5V DC input to +1.8V output.
>
> In both cases, all three voltages are available.
>
> What are the pros and cons?
>
> Also, in the Olimex schematic (top right corner), I see a Schottky Barrier
> being used (BAT54C),http://www.olimex.com/dev/images/ARM/ATMEL/SAM9-L9260-REV-B-sch.gif
>
> I believe the main purpose was to isolate the +5V, +3.3V and +1.8V power
> rails. Is this really required?
>
> Thanks,
> Saritha.

Both of your questions will be answered when you determine what power
sequencing you need.  The 1st when you know the power-up sequence, and
the 2nd when you know the power-down sequence.

G.

On Fri, 13 Jun 2008 18:08:26 -0500, "kalyanamsaritha"
<kalyanamsaritha@gmail.com> wrote:


>Also, in the Olimex schematic (top right corner), I see a Schottky Barrier
>being used (BAT54C),
>http://www.olimex.com/dev/images/ARM/ATMEL/SAM9-L9260-REV-B-sch.gif
>
>I believe the main purpose was to isolate the +5V, +3.3V and +1.8V power
>rails. Is this really required?

It's probably there to protect the regulators from being "upside down"
when securing the device. That is, if the input of the regulators gets
shorted to ground when pulling out the power plug, the capacitors on the
output (regulated) side of the regulator will discharge back through the
regulator to the input. That's a Bad Thing. IIRC there is a small
internal diode to accommodate some transients but a larger, external
diode gives a bit more protection.

-- 
Rich Webb     Norfolk, VA

>Both of your questions will be answered when you determine what power
>sequencing you need.  The 1st when you know the power-up sequence, and
>the 2nd when you know the power-down sequence.
>
>G.

Here is some info on power-up sequencing ...

<< snip >>

There is no specific sequence to power up an AT91 microcontroller. Either
the core voltage or I/O voltage can be powered first without any risk of
destruction.

However, Output values and power consumption are not guaranteed during
power up sequence. We advise to perform a product specific reset after
stabilization of supplies to have a known state.

Although only one voltage rail can be supplied without any risk of
destruction, correct functionality of the device cannot be guaranteed.