Thursday, April 7, 2016

Regulator stops working when ENABLE volts drops below 12.5v

Today I received a comment from Jeremy that during bench-testing he noted his regulator would stop functioning when the ENABLE voltage drooped below 12.5v.  Specificity, the Field FETs would not turn on.

Upon reviewing things, I think the issue is in U3 (the FET Driver).   The IRS21850 has an undervoltage cut-out of nominally 8.2v - but can be as high as 8.9v per the spec sheet.   Combined with other worst-case  voltage drops of D16 and Q1, we think this is the issue.

A good corrective action will be to replace U3 with a different driver chip.  The IRS21867 is a dual driver chip (High and low side), but its high-side is pin compatible with the existing PCB layout.   Plus, its under voltage cutout is 4.5v - much more in line with the FETs needs.  So, a proposed errata for v0.1.4 of the PCBs is:

  1. Replace U3 with IRS21867 driver ship (Mouser URL:  IRS2186
  2. Solder bridge U3 pins 3 & 4 (this will pull the unused low-side drivers input to a fixed state)

I have updated the Mouser BOM (link here) to include BOTH the old and the new driver chips.  When ordering you can select which one you wish to use.  Once I am able to verify the function with the IRS2186, I will remove the original IRS21850 option.

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