bq77PL900 Cell Connection Order

The customer should use a sequence which does not exceed the device ABS MAX values during cell connection. If the ABS MAX is exceeded the device may be damaged. A common damage is shorted inputs possibly from excessive differential voltage between adjacent VCx pins.

Waking the part during cell connection should be avoided since the part will begin monitoring the un-connected cells and could induce a voltage above ABS MAX if the voltage range of the connected cells is sufficient.  The part may wake unexpectedly due to capacitance across the FETs, FET leakage, fixture resistances or other leakage paths.  A load and/or large capacitance on the pack terminals during assembly may help prevent turn on.

When cells are connected randomly, assuming the input filter capacitors were initially discharged, they will distribute voltage between the connected cells. If the intermediate connections are made without other disruption, the voltages are approximately the same and no damage should result. However if fixture loading, assembly tooling, or device operation pulls the inputs to an excessive voltage, damage could result. Some examples may help explain the risks, assuming 3.6V cells.

1. An operator connects every other cell.  The input terminal for a cell between two others accidentially is touched to the upper or lower cell, so that input gets 2x normal voltage.  3.6 x 2 = 7.2V which is less than the 8V ABS MAX voltage, no damage is expected.

2. An operator connects cell 2 and cell 7. the cell 3 input is accidentally touched to cell 6 or cell 7.  The voltage applied to the cell 3 inputs is 4 x 3.6 = 14.4 or 5 x 3.6 = 18 V. Either voltage exceeds the ABS MAX for the input and the part is likely damaged.

3. An operator connects cells 1 and 9.  The part wakes up and begins loading the inputs.  Although the capacitors initially distributed the intermediate cell voltages evenly, they do not hold a DC value and eventually some input exceeds its ABS MAX.  If this input shorts, another may exceed the maximum.

The EVM connection sequence was believed to be the best for its on board connections and may be suitable for other systems. The customer's suggested connection order may be suitable for their board design and assembly process.

Please note that with the simple capacitor configuration shown in the datasheet and implemented on the EVM it may be difficult to connect cells while meeting the VC11 ABS MAX limits.  Alternate capacitor configurations may be beneficial such as connecting the bottom VC10 capacitor to ground and connecting a separate capacitor between VC11 and ground.

Hi,

Is advice also the same as when cell connection order is the following?

1. Connect BAT+

2. Connect GND

3. Connect Vc1 - Vc11 with abitrary order

Regards

Yes. Since the device references signals to its GND pin which is normally connected to Battery-, this would be recommended first.  However any sequence which does not exceed absolute maximum for the device should be OK.