BQ40Z50-R2 Datasheet, PDF (30/51 Pages) Texas Instruments – 1-Series, 2-Series, 3-Series, and 4-Series Li-Ion Battery Pack Manager

bq40z50-R2
SLUSCS4 – JUNE 2017
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Capacitors C1 and C2 help protect the FETs during an ESD event. Using two devices ensures normal operation
if one becomes shorted. To have good ESD protection, the copper trace inductance of the capacitor leads must
be designed to be as short and wide as possible. Ensure that the voltage rating of both C1 and C2 are adequate
to hold off the applied voltage if one of the capacitors becomes shorted.
Figure 22. bq40z50-R2 Protection FETs
9.2.2.1.2 Chemical Fuse
The chemical fuse (Dexerials, Uchihashi, and so forth) is ignited under command from either the bq294700
secondary voltage protection IC or from the FUSE pin of the gas gauge. Either of these events applies a positive
voltage to the gate of Q5, shown in Figure 23, which then sinks current from the third terminal of the fuse,
causing it to ignite and open permanently.
It is important to carefully review the fuse specifications and match the required ignition current to that available
from the N-channel FET. Ensure that the proper voltage, current, and Rds(on) ratings are used for this device.
The fuse control circuit is discussed in detail in FUSE Circuitry.
4P
to 2^nd Level Protector
to FUSE Pin
Figure 23. FUSE Circuit
9.2.2.1.3 Li-Ion Cell Connections
For cell connections, it is important to remember that high current flows through the top and bottom connections;
therefore, the voltage sense leads at these points must be made with a Kelvin connection to avoid any errors due
to a drop in the high-current copper trace. The location marked 4P in Figure 24 indicates the Kelvin connection of
the most positive battery node. The connection marked 1N is equally important. The VC5 pin (a ground reference
for cell voltage measurement), which is in the older generation devices, is not in the bq40z50-R2 device.
Therefore, the single-point connection at 1N to the low-current ground is needed to avoid an unwanted voltage
drop through long traces while the gas gauge is measuring the bottom cell voltage.
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