Wednesday, March 07, 2012

IC Design Interview 3: Bandgap Reference

Life has intruded so not much update on the boost converter. I do now understand there is a brand new shiny zero that has to be taken care of. I also now understand the gate drive needs to forced to oscillate at some duty cycle regardless of the output value. I need to rework the feedback compensator to account for the boost AC characteristics instead of the buck that is in there now. Until I have that ready, here's some more interview notes.

The bandgap equations must be derived sometime during every integrated circuit design interview. At least it seems that way. It is a standard, useful circuit found in nearly every chip. I think the discussions about the circuit are better able to gauge a candidate's knowledge and experience, though. 

Start with a schematic like the one shown below. 

That last equation isn't the final answer, though I've never been asked in an interview to go beyond it. VBE has a negative temperature coefficient and the VT has a positive temperature coefficient so appropriate values of resistors and bipolar multipliers can be found.In general R1 and R2 are made equal. Then the bipolar multipliers are set based on layout. Pick a rectangular array that allows Q1 and Q2 to be arranged in a common centroid configuration. Then set the ratio of R1 and R2 for zero temperature coefficient.

There are at least four other sub-topics that I will put off until a later blog entry.

  • Current conveyor instead of an op-amp
  • Lower voltage reference
  • Producing a VPTAT (Voltage Proportional To Absolute Temperature)
  • Start-up issues
Bruce McLaren