BQ76940: for 7 cells application

Hi Steve,

The bq76940 will work with a minimum of 9 cells, that is due to minimum operating voltage, biasing, and communication between the cell groups.  If you connect a bq76940 with only 8 or fewer cells as would be intended for a '930, you will have a permanent XREADY fault and will not get all cell voltages.  Those from cells 5-8 will have dormant values as the part tries to get voltages for the higher cells.

If the construction of your board allows separating the high current path from the cell inputs, one way for a simple bench test would be to configure the '940 for 10 or 11 cells for simulating 7 or 8 cell operation respectively.  Connect the top group inputs as if there were 3 cells. Connect your bottom 7 or 8 board cell inputs to their source and the high current path to the top used cell input. Again this only works if the high current path is not connected to the top board cell input. Now connect a power supply with resistor dividers for 3 cells across the top group's 3 cell inputs and set it to something between your UV and OV limits. Be sure to provide a pull down for TS3 to VC10x.  The part should boot and operate with the 7 or 8 cells.  Ignore the registers for the upper cells. With the power supply set for cell voltages between UV and OV the upper group will not provide a UV or OV fault.

A better method would be to replace the bq76940 monitor IC on the board with a bq76930.  You may have noticed that the bq76930EVM and bq76940EVM share the same board which is selectively populated for the installed monitor.  If your board is assembled for the '940 it will have extra parts, but the '930 should work fine if pin 1 is aligned on the board pattern.  You will likely need to connect the board cell inputs which feed the filters for VC10 and VC15 together so that the REGSRC voltage follower FET will get power.  Also be sure that with the lower voltage the REGSRC voltage follower FET is not starved for current, you may have to change the drain resistor.  When doing this connection remember that if your board had transient protection it was likely selected for the higher voltage.