Spehro Pefhany wrote:
> What is that supposed to mean? None of the high-side drivers presented
> are short-circuit proof.
>
>>  Note that the MMTB4403 can only handle 500mV
>> accross it at 250mA.  This circuit will also be quite slow turning
>> off, but that may not matter.
>
> ?? 'Handle 500mV accross' it?   You talking about SOA? Static Pd(max)
> at Ta?

You specified MMTB4403, which comes in a SOT-23 package.  At 25C ambient the
junction will reach 150C at 350mW.  That is the absolute maximum, and I was
using the more "normal" dissipation guideline of 1/8 W or 125mW for a SOT-23
for general practise.  The current spec is 250mA.  125mW / 250mA = 500mV,
which is the most voltage drop the transistor should therefore be subjected
to when on.  Even at the 350mW guaranteed to fry threshold, you only get
1.4V.

This is so far from short-circuit proof that I thought it was worth pointing
out.  For example, if the load limited its voltage to 10V at 250mA, the
transistor would dissipate 500mW, which is in the guaranteed to fry range
even though it's far from what most people would consider a short circuit.

> 'Quite slow' turn-off? tf will be << 1usec.

I was trying to point out that the turnoff of the circuit would be much much
slower than what you might otherwise expect from these transistors.  Both
transistors will be saturated when on, and the relatively high resistances
of 10Kohms and 6.8Kohms will not quickly force the transistors to turn off.

Neil never said what kind of load he was driving or how fast or how often he
needs to switch it, so we don't have enough information to know whether any
of this matters, which is why I pointed it out.

In any case, it sill makes sense to get rid of the R3 and put R1 between the
emitter of Q1 and ground.


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