Xiaofan Chen wrote:
> Actually I see some problems with the selection of components in Olin's
> schematics. Maybe they are not a big problem for hobbyists though.
>
> 1) Voltage rating of C1/C2/C3 is only 35V. This may be a bit low if the
> input voltage is really 30V with big ripples.

But it's not.  The voltage is specified as 30V maximum, including any ripple
from an AC power source.  When used with the recommended wall wart in the
US, that rail will be at about 25V.

> 2) I'd like to see a fuse in the input and a zener (over voltage
> protection) after
> the buck converter to protect the other parts.

After the buck converter?  I don't see the point since the job of the buck
is to make about 5.5V.  Unless it physically breaks, the output won't exceed
6V, which is the maximum input rating for the 5.0V linear regulator IC1.

It is the user's responsibility to ensure the input voltage stays within
specification.  Like any specification, you can't violate it and then
complain if the unit breaks.  A line has to be drawn somewhere.  No amount
of circuitry can guarantee it won't break at all possible voltage levels.
The question is therefore what is a good tradeoff between allowing a wide
input voltage range versus the cost of supporting it while maximizing
lifetime profits for the product.  There is no way to know the right answer,
but I do think extra protection circuitry would not be worth the very rare
case when it would be useful.  Customers are usually not willing to spend a
lot of money to protect themselves from their own stupidity or from rare
part failures.

One exception is the optional clamp circuit on page 1.  This is intended for
automotive 12V supplies where large voltage spikes do occur regularly beyond
the end user's control.  To keep the cost reasonable and allow a wider input
voltage range for the normal case, this clamp circuit is unpopulated but the
pads are provided.

> 3) As Russel said, normally there will be a small series gate resistor
> is good to have.

Yes, this is a common religious conviction.  Think about *why* such a series
gate resistor is often used and you can see that it's not necessary in this
case.  The edges on R4 just can't be that fast, so even if Q2 and Q4 have
infinite gain there won't be a problem.


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