BQ27750 Datasheet, PDF (24/34 Pages) Texas Instruments – Impedance Track Battery Gas Gauge and Protection Solution for 1-Series Cell Li-Ion Battery Packs

bq27750
SLUSCM7 – JUNE 2017
10 Layout
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10.1 Layout Guidelines
• The layout for the high-current path begins at the PACK+ pin of the battery pack. As charge current travels
through the pack, it finds its way through protection FETs, a chemical fuse, the Li-Ion cells and cell
connections, and the sense resistor, and then returns to the PACK– pin. In addition, some components are
placed across the PACK+ and PACK– pins to reduce effects from electrostatic discharge.
• The N-channel charge and discharge FETs must be selected for a given application. Most portable battery
applications are a good option for the CSD17575Q3. These FETs are rated at 60-A, 30-V device with Rds(on)
of 1.9 mΩ when the gate drive voltage is 10 V. The gates of all protection FETs are pulled to the source with
a high-value resistor between the gate and source to ensure they are turned off if the gate drive is open. The
capacitors (both 0.1 µF values) placed across the FETs are to help protect the FETs during an ESD event.
The use of two devices ensures normal operation if one of them becomes shorted. For effective 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 these capacitors is adequate to hold off the applied voltage if
one of the capacitors becomes shorted.
• The quality of the Kelvin connections at the sense resistor is critical. The sense resistor must have a
temperature coefficient no greater than 50 ppm in order to minimize current measurement drift with
temperature. Choose the value of the sense resistor to correspond to the available overcurrent and short-
circuit ranges of the bq27750 gas gauge. Select the smallest value possible in order to minimize the negative
voltage generated on the bq27750 VSS node(s) during a short circuit. This pin has an absolute minimum of
–0.3 V. Parallel resistors can be used as long as good Kelvin sensing is ensured. The device is designed to
support a 1-mΩ to 3-mΩ sense resistor.
• A pair of series 0.1-μF ceramic capacitors is placed across the PACK+ and PACK– pins to help in the
mitigation of external electrostatic discharges. The two devices in series ensure continued operation of the
pack if one of the capacitors becomes shorted. Optionally, a transorb, such as the SMBJ2A can be placed
across the pins to further improve ESD immunity.
• In reference to the gas gauge circuit the following features require attention for component placement and
layout: Differential Low-Pass Filter, I²C communication, and power backup input (PBI).
• The bq27750 gas gauge uses an integrating delta-sigma ADC for current measurements. Add a 100-Ω
resistor from the sense resistor to the SRP and SRN inputs of the device. Place a 0.1-μF filter capacitor
across the SRP and SRN inputs. Optional 0.1-µF filter capacitors can be added for additional noise filtering
for each sense input pin to ground, if required for your circuit. Place all filter components as close as possible
to the device. Route the traces from the sense resistor in parallel to the filter circuit. Adding a ground plane
around the filter network can add additional noise immunity.
0.1 µF
0.1 µF
100
100
0.1 µF
0.001, 50 ppm
Sense
resistor
Ground
Shield
Figure 22. bq27750 Differential Filter
Filter Circuit
• The bq27750 has an internal LDO that is internally compensated and does not require an external decoupling
capacitor. The PBI pin is used as a power supply backup input pin, providing power during brief transient
power outages. A standard 2.2-μF ceramic capacitor is connected from the PBI pin to ground, as shown in
application example.
• The I²C clock and data pins have integrated high-voltage ESD protection circuits; however, adding a Zener
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