How does the circuit contribute to 1.235V reference?

I am studying a reference cicuit,but i cann't understand a part that is remarked with red frame in the schematic.Please senior designer help me analysis and explain to me about the circuit in the red frame. Thanks!

Re: Reference Question

Hi my all teachers:
Please talk your viewpoints, i am very urgent and confused.Thank you very much!

Re: Reference Question

Circuit in Red block will maintain 1.235 V at the output by maintaining the common mode level of npn transitors in the reference generation block. If you notice the connection to the bases of the input are having a resistor drop.Hence they will maintain a level at the input and at the emitter of the output transistor.

Mostly, this will work as a start up to the internal nodes of the blue block. If you want to verify it, remove this block and replace it with equivalent voltage source. Then try and ramp up the supplies to see whether it makes any difference.

I hope that I am correct here and it helps................

Re: Reference Question

The block may be used for thermal and star-up circuit for bandgap.

It protects the power transistor(pass transistor).

The circuit may be a LDO.

There are bandgap, protection circuit and star-up ciruit blocks.

Re: Reference Question

Thanks the upper two teacher 's direction! But i needs a formula that can explain the effect of red frame,so i can clearly understand that problem.

Re: Reference Question

i do not think the circuit in red is meant for start-up, as mentioned by a previous poster. the resistors are good enough to ensure startup.

and i am perplexed that someoone mentioned thermal protection?

there are a few flaws with the circuit which i think u may need to check:
1. supply voltage of 2.5V ain't going to work for this circuit, try -60C.
2. the circuit is not compensated ?!

Finally, abt the question. I am making a guess.

The circuit in red should be normally off to ensure that the output is BG (Vbe + Ratio R.ΔVbe). R55 should be small. The ckt is there possibly to act as a fast discharge of REF in the event that the REF overvoltage. This could be due to supply spiking. It's connected in a differetial manner perhaps to increase noise immunity.

As I do not have your value for the resistors and BJTs' areas, it's kind of difficult to know exactly what is it doing. It's a tough job to ensure that the BJTs are off as u need to take into consideration the change of Vbe with temp.

Run some sims, with and without the part in red and see what u get, that's the easiest way to learn. I would like to learn what u get for ur result also.

Re: Reference Question

Hi all my teacher:
I have edited that circuit again.Now every device have been marked the parameter in the schematic. I hope you can simulate that circuit in your free time and help me analysis.Thanks!
I express a little my viewpoints about lastdance' post:
1. For 2.5V supply voltage,i have simulated the circuit at -60C and found the
value of Vo_1.235V out is around 1.224V.the circuit is working ,but the value
of reference out is lower than the typical 1.235V.
2. It has a small compensation capacitor in the circuit.Pls u see.
3. The circuit of the red block is off at normal operating .Your guess is right.
4. I hear someone say the red block is thermal compensation.But they cann't
use a quantitative formula to explain,and i am perplexed.Do you agree their
viewpoint?