Please help me to understand this schematic....

[nnvkct]

Here is the circuit:

And I'm stuck on following questions:

- If P3.4 =0 P3.5 =0, or P3.4 =1 P3.5 =1, or P3.4 =1 P3.5 =0, or P3.4 =0 P3.5 =1, what's the value of VDD and VPP.
- What's the role of zener 5.6V and zener 12V on this circuit.

Please help me. Any ideas would be great!

(of course, I tried to stimulate this circuit by Electronics Worldbench but I'm confused its result. Please help me to understand how does it run.)

 

[artem]

roughly if VDD is more that 12.6 V , Vpp can take based on P3.4 and 3.5 :
- 0V VP3.4 = ~ 1V , VP3.5 irrelevant
- 5V VP3.4 = 0 V, VP3.5 more than ~1 V
- 12 V VP3.4 = 0 VP3.5 = 0

 

[gszczesz]

Assuming Vdd > 12.6V, this circuit is a voltage regulator.

P3.4 is the shutdown. If P3.4 is high, then the base of Q1 is brought down to 0.

P3.5 is the voltage select pin: 12V on the output if P3.5=0, 5V on the output if P3.5>1.

The 12V zener sets the base voltage of the output follower (Q1) to be 12V, but you need an extra Vbe for biasing. Therefore D3 is added to take out the extra Vbe.

The 5.6V Zener already includes an extra 0.6V, so it does not need an extra Vbe.

In summary, this is a 5V or 12V output regulator, with P3.5 the select pin, P3.4 the shutdown pin.

Greg