BQ76200: fail to pass the short-circuit test

Hi Yuan,

From the waveform it looks as if the FETs may have failed during the test.  It will be important to see if the BQ76200 tried to switch and was unsuccessful or if the command to switch was not sent.  You may have more details about the circuit condition and operational status after the test which may be helpful in your investigation.

Looking at the schematic, the BQ76200 looks reasonable.  Good things to check would be if C55 were substituted with a large capacitor.  Checking DSG_EN during the event would show if the system intended to close the path.

On the BQ76940 circuit, see the application note http://www.ti.com/lit/pdf/slua749 section 3.  The Rf resistors appear to be 2k ohm such as R52 R53 and the Cf capacitors seem to be 1 uF such as C20.  With the BJT balancing path the base resistor will conduct with the diode during short circuit, so the base resistors will draw current from the power filter caps.  Check VC5X during SCD to see that it is staying above VSHUT.  Inspecting VC5X, CAP1, and DSG_EN along with the current during the SCD event may be informative.  If CAP1 collapses during SCD the BQ76940 may not be sending the DSG low to tell the BQ76200 to turn off.  If this is the issue, shorten the SCD delay, increase Cf capacitor size, and/or increase the base resistance for the BJTs so VC5X remains above VSHUT.