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Why does the bq77910 EVM have a TVS diode at D18? Do I need one in my design?
Do I need clamp diodes on the inputs?
Which inputs are most sensitive?
The bq77908 and bq77910 have 2 absolute maximum ratings on the inputs. One is the differential voltage, the other the single ended voltage. The differential limit will be most important in test or connection where a fixture may load one input while a voltage is applied to another. In quiescent operation the cells will provide a stable voltage under this limit. Balancing will raise the input differential voltage of the cells adjacent to the one balancing. The single ended limit can be a concern in test and assembly, but also in operation due to inductive transients. The input filter and system transient protection must keep the inputs within their abs max ratings.
The device system engineers suggest a clamp diode on the top used VCx input appropriate for the cell count to supplement the system level transient suppression. This is D14 or D12 for VC1 on the EVMs. Anything coming through the filter will be limited at the top cell input, the capacitors will form a divider and tend to hold the lower inputs down. This arrangement is attractive since there is only one diode and any leakage in the diode is common to all cells. In early testing with the EVM it was observed that with some cells, the 4th cell input could be damaged in short circuit testing, so D18 was added.
The EVM protection scheme may not be suitable for all systems. The technique may be undesirable, the protection may be OK, too much or too little. The effect the system must protect against is V = L x dI/dt. Inductance 'L' will be largely fixed by the cells but the interconnect construction will contribute. Current 'I' will vary with the cell and stack design. The 'dt' switching time will depend both on the system switch and the protector switch.
The input resistors provide both the filtering with the capacitor and the balance current. With a circuit like the EVM looking to use the maximum internal balance current, the resistors are small and the filter may not suppress the transient much. Clamping will be more important to keep the part safe. With larger resistors for lower balance current, the filter is more effective and less clamping may be needed.
The datasheet shows a single ended absolute maximum for the inputs as 5V per cell. The internal circuit implementation may vary for the inputs and leave more margin for some inputs than others. This does not imply that the absolute maximum can be disregarded, always check the latest datasheet for device limits. The following table shows the most sensitive pins and may help the user understand why some pins are damaged first in development testing.
Cell number, bottom up
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