BQ76940: BQ76940

Hi User,

I did not design the circuit. 

For the casual reader, the questions from the attachment concern tiduc43a, TIDA-01093, specifically the load removal circuit of figure 7.

" TDELAY is the time from the moment when the load is removed to the next moment when

the voltage VLOAD goes to the voltage below 0.5 V again，Why is there a delay?

The load removal event can then be detected by the MCU, Is LOAD_RM_DET signal configured to pull or drop-down ?

If the battery is 16 s, Is D21,D24,D28,D33 steady voltage equal to the total battery voltage? How to choose?

Can D24, D28, D33 replace it with a zener diode ?

Why D21 package is SMB,SMA is ok?

Thanks for your reply! " 

1. There is a delay between removal of the load and VLOAD (net LOAD_RM_DET) going low because there is no direct pull down on the gate of signal FETs Q15 and Q19.  It is the leakage current through the zener diodes which will discharge the gates.

2. "...Is LOAD_RM_DET signal configured to pull or drop-down ?" Yes.  With PACK- pulled up Q15 is on as a source follower pulling up the source near VCC_B.  When PACK- drops suddenly the capacitance of the diodes will push the gates down and Q19 will go low.  If PACK- drops slowly the leakage of the zeners will pull down the gates and turn off Q15,  LOAD_RM_DET may drop slowly, times may be long as shown in table 6.

3. The intent of the circuit is described in section 2.3, the author wanted high impedance during normal voltage operation, so the zeners will not normally conduct fully.  If the voltage does increase to conduction current is limited by the 8.2M resistor to keep current at an acceptable level.  At higher voltages the step size of the available zener diodes for D21 may be large, the values for D24, D28, D33 may be adjusted to keep the total voltage at an acceptable level while not exceeding the gate voltage limit of Q19.  Select values for your design to protect the gate while keeping current in an acceptable level with component tolerances.

4. Yes, D24, D28 and D33 can be replaced with a single zener diode if a suitable voltage value is available, however this will limit the amount that the gate of Q19 could be pushed below BAT-.  Depending on selection of Q19 you may also use back to back zeners to allow a negative voltage larger than one diode Vf but still removing a component.

5. D21 is a SMB diode since it was the package available at the time of the design.  At higher voltages for a zener diode a normal test current may result in a large power value and large package to handle a reasonable power limit.  In this circuit current is kept low so the power is low and an SMA or smaller package can be used if available.  Always check the power and component ratings in your final design.