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Design >> Analog Design >> BGR correctness
https://designers-guide.org/forum/YaBB.pl?num=1357293122

Message started by james_lanren on Jan 4^th, 2013, 1:52am

Title: BGR correctness
Post by james_lanren on Jan 4^th, 2013, 1:52am

I have designed a bgr which has an output voltage against temp as follows. The output voltage is 1.15 instead of 1.205. Will it be tolerable?
Also the curvature is upward facing, is that correct?

Title: Re: BGR correctness
Post by yvkrishna on Jan 4^th, 2013, 8:01am

Hi James,

I assume this is simple BGR design without any curvature compensation used.

o/p voltage will not exactly be 1.205 for every process(depends on lot of things) but usually close it if you design a classical voltage bandgap.

Tolerance is always about the total variations with PVT, absolute value is usually easy to trim and control.

And about curvature facing upwards, it might be to do with the BJT used, did you try to observe if BJT alone has proper temperature characterless.(CTAT nature)
use a different bjt/diode and double check.

you can post your schematic for better replies to your queries.

Regards,
yvkrishna

Title: Re: BGR correctness
Post by Dan Clement on Jan 4^th, 2013, 10:34am

This can be normal, it just depends on the tempcos of resistors, offsets, etc.

It could also be a sign that your collector current is too high...

Title: Re: BGR correctness
Post by loose-electron on Jan 12^th, 2013, 4:11pm

suggest posting the schematic. Also have you modeled in you mismatches and offsets? Those can throw the accuracy off a lot more than the tempco variance.

Title: Re: BGR correctness
Post by analog_wiz on Jan 14^th, 2013, 4:56am

Have you checked the leakage of the startup circuit at high temp?

Title: Re: BGR correctness
Post by RobG on Feb 5^th, 2013, 1:28pm

A bit late, but something is very messed up. Your curvature is 75 mV and upward. 4 mV and downward is typical.

Title: Re: BGR correctness
Post by Erez_Sarig on Feb 7^th, 2013, 1:04pm

Hi,

It looks like you are very close to fine tune the BGREF your PTAT is too high so decrease it and you should get a better level.
Best thing to do is to take aside the BJT and using ideal devices see the real ratio of PTAT and CTAT. also measure the PTAT which should be around 166uV/C.
Still it is hard to conslute without seeing the schematic....

Erez