BQ27742-G1 Datasheet, PDF (38/46 Pages) Texas Instruments – Single-Cell Li-Ion Battery Fuel Gauge with Programmable Hardware Protection

bq27742-G1
SLUSBV9C – MARCH 2014 – REVISED FEBRUARY 2016
10 Layout
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10.1 Layout Guidelines
10.1.1 Li-Ion Cell Connections
For highest voltage measurement accuracy, it is critical to connect the BAT pin directly to the battery terminal
PCB pad. This avoids measurement errors caused by IR drops when high charge or discharge currents are
flowing. Connecting right at the positive battery terminal with a Kelvin connection ensures the elimination of
parasitic resistance between the point of measurement and the actual battery terminal. Likewise the low current
ground return for the fuel gauge and all related passive components should be star-connected right at the
negative battery terminal. This technique minimizes measurement error due to current-induced ground offsets
and also improves noise performance through prevention of ground bounce that could occur with high current
and low current returns intersecting ahead of the battery ground. The bypass capacitor for this sense line needs
to be placed as close as possible to the BAT input pin.
10.1.2 Sense Resistor Connections
Kelvin connections at the sense resistor are just as critical as those for the battery terminals themselves. The
differential traces should be connected at the inside of the sense resistor pads and not anywhere along the high
current trace path in order to prevent false increases to measured current that could result when measuring
between the sum of the sense resistor and trace resistance between the tap points. In addition, the routing of
these leads from the sense resistor to the input filter network and finally into the SRP and SRN pins needs to be
as closely matched in length as possible else additional measurement offset could occur. It is further
recommended to add copper trace or pour-based "guard rings" around the perimeter of the filter network and
coulomb counter inputs to shield these sensitive pins from radiated EMI into the sense nodes. This prevents
differential voltage shifts that could be interpreted as real current change to the fuel gauge. All of the filter
components need to be placed as close as possible to the coulomb counter input pins.
10.1.3 Thermistor Connections
The thermistor sense input should include a ceramic bypass capacitor placed as close to the TS input pin as
possible. The capacitor helps to filter measurements of any stray transients as the voltage bias circuit pulses
periodically during temperature sensing windows.
10.1.4 FET Connections
The battery current transmission path through the FETs should be routed with large copper pours to provide the
lowest resistance path possible to the system. Depending on package type, thermal vias can be placed in the
package land pattern's thermal pad to reduce thermal impedance and improve heat dissipation from the package
to the board, protecting the FETs during high system loading conditions. In addition, it is preferable to locate the
FETs and other heat generating components away from the low power pack electronics to reduce the chance of
temperature drift and associated impacts to data converter measurements. In the event of FET overheating,
keeping reasonable distance between the most critical components, such as the fuel gauge, and the FETs helps
to decrease the risk of thermal breakdown to the more fragile components.
10.1.5 ESD Component Connections
The ESD components included in the reference design that connect across the back-to-back FETs as well as
from PACK+ to ground require trace connections that are as wide and short as possible in order to minimize loop
inductance in their return path. This ensures impedance is lowest at the AC loop through the series capacitors
and makes this route most attractive for ESD transients such that they are conducted away from the vulnerable
low voltage, low power fuel gauge and passive components. The series resistors and Zener diodes connected to
the serial communications lines should be placed as close as possible to the battery pack connector to keep
large ESD currents confined to an area distant from the fuel gauge electronics. Further, all ESD components
referred to ground should be single-point connected to the PACK– terminal if possible. This reduces the
possibility of ESD coupling into other sensitive nodes well ahead of the PACK– ground return.
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