how to clip a voltage to 3.3V by switching transistor such a

[blue sky]

The board have a 3.3v refence voltage. Another input voltage can
change from 1.5V to 5.5V, but need to clip to 3.3v.
Vout =Vin (Vin <3.3V)
Vout =3.3v (VIN >=3.3V)
If I want to use Infineon's switching transistor (SMBT3904/SMBT3906),
how to design it? I know its a simple question, I'm just a beginer in
hardware design.
Any tips is welcome.

 

[Jamie]

Vin to the Collector of the 3906, Emitter will be the output.
resister from Collector to base aprox 470 ohm. a zener diode of
3.0 Volts @ 1 watt cathode at base, anode at ground side.
the average voltage drop in get via the Be (Base Emitter) will give you
aprox .5 or less voltage drop.
so 1.5 input may cause aprox 1.0 output. and any input from 3.0 volts up
t0 5 o 6 volts should maintain to ~ 3.3v.
P.S.
they make Dc-Dc convert Ic's that will hold at selected voltage
even when the input is below the desired output level.


blue sky wrote:

The board have a 3.3v refence voltage. Another input voltage can
change from 1.5V to 5.5V, but need to clip to 3.3v.
Vout =Vin (Vin <3.3V)
Vout =3.3v (VIN >=3.3V)
If I want to use Infineon's switching transistor (SMBT3904/SMBT3906),
how to design it? I know its a simple question, I'm just a beginer in
hardware design.
Any tips is welcome.