>> >> The load will be up to ~250mA, btw.
>> >
>> > Use Bob's method only. Eg.
>> >
>> > R1 1K 1/4W
>> > R2 10K
>> > R3 6K8
>> > NPN: 2N/MMBT4401
>> > PNP: 2N/MMBT4403
>>
>>You had better make sure the load will use all its
>>available voltage else
>>the PNP transistor will fry.
>
> What is that supposed to mean? None of the high-side
> drivers presented
> are short-circuit proof.

I took it to mean (possibly wrongly) that the PNP needed to
be in saturation or close to it.

Without looking back I don't know if the 250 mA was
specified when those resistor values were given. As will be
seen below, the circuit is in trouble as shown at 250 mA
with that transistor and resistors. It can almost be
redeemed but ideally either a higher current gain transistor
for Q2 or even better a MOSFET would make it work well.

Because:

Drive current will be about (12v-Vbe_Q2)/R1 = 11.4/10k = 1
mA+
For 250 mA load the beta of Q2 wants to be 300 or better.
Beta of an xx4403 is under 100 guaranteed at 250 mA so you
need say 3x as much base drive for Q2 if you want to support
250 mA.
R2 = 3k3 or 2k7.
E&OE check the sums.
Dissipation in R2 is still minimal.

Vce_sat max can be well over 1 volt for this device at 250
mA so Pd at 250 mA = > 250 mW.
That would get the TO92 part hot (2N4403) and the SMD part
very hot (MMBT..)

If I was doing this I'd start with my favourite jellybean
BC327-40.
500mA to 700 mA rated depending on manufacturer.
The -40 means mean beta in batch of 400.
(~= Geometric mean of 250 to 600 range).
This is good for most such tasks.
You'd need to work through the above example to be sure.

An easier way to run cool is to use a FET.
Either a high side P Channel or a single N channel if you
can tolerate the low side drive to the load.

Type depends on drive voltage.
The beautiful but not too common in the US CES2310 that I
mentioned a while ago would do it with ease.
Failing that the FDN337 with Rdson = 0.082 typ at 2.5V
drive.
Headline Rdsons are always a lie compared to DC steady state
but at 3V gate you'd get well under 0.1V on voltage at 250
mA for 25 mW dissipation.
With Tja of 250 C/W it would run touch cold.
Probably :-)

$US0.44/1 Digikey
$US0.12/1000.
A bargain, if you can't get CES2310's :-)

        http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=FDN337NCT-ND

        http://www.fairchildsemi.com/ds/FD/FDN337N.pdf

It's FDN338 P Channel stablemate is also your friend at
about the same price.
As expected, not quite as good an Rdson as N channel part,
but still very good in this task.

        http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=FDN338PCT-ND

        http://www.fairchildsemi.com/ds/FD/FDN338P.pdf

Use this in place of Q2 in the original circuit and Bob's
your uncle.
R1 10k or whatever.
R2 100k or  >>> R2.
R3 almost anything.
Speed gets slower with higher R.
A speedup cap and some R tailoring could make this very fast
if required (for some values of very).

>>  Note that the MMTB4403 can only handle 500mV
>>accross it at 250mA.

A valid concern - as above.
Beta won't support required current with provided drive



            Russell McMahon


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