bandgap generator at low supply

[pvreddy]

hi all,

Gve me an idea to generate a bandgap reference voltage from low supply voltages, say 1.6v. Is it possible to generate bandgap ref. voltage from 1.2v supply?

 

[jecyhale]

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Ｗｈｙ　ｎｏｔ？

 

[rfsystem]

In conventional bandgap topologies the minimum voltage is

VBE+a*VT+VDSAT

It is the series path of the parasitic PNP, a kT multiplier and a PMOS regulator current output.

In a non-curvature compensated band the absolute temperature coefficients of

VBE and a*VT should cancel

dVBE/dT~-2mV/K

dVT/dT=+(1/300)*26mV

so a solves to about 23

The minimum is therefore somewhere

700mV+23*26mV+150mV=1448mV

The interesting part is now if VDD is less than this what should we do?

Simple

Create a circuit which add 1/2 of VBE to 23/2 of VT together. That is an exercise for an opamp course.

Made it step by step:

1. Generate kT current source and VBE reference
Minimum voltage here VBE+VDSAT (850mV guess)

2. Dividing VBE by 2 with two relative high ohmic resistors

3. Connecting the VBE/2 to the noninverting opamp

4. Feeding a NMOS replicated kT current source to the inverting input

5. Using a feedback resistor so that (a/2)*VT is across

 

[elantra]

it's possible, but, if you run the simulations under corner, be careful, some cases, the performance will be quite bad if you need good transient performance, good accuracy, quick start-up,etc. You know, with low supply, some fets will work abnormally.