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BQ27743 Arkusz danych(PDF) 37 Page - Texas Instruments |
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BQ27743 Arkusz danych(HTML) 37 Page - Texas Instruments |
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37 / 42 page  37 bq27743 www.ti.com SLUSCM0A – JUNE 2017 – REVISED SEPTEMBER 2018 Product Folder Links: bq27743 Submit Documentation Feedback Copyright © 2017–2018, Texas Instruments Incorporated 11 Layout 11.1 Layout Guidelines 11.1.1 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. 11.1.2 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. 11.1.3 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. 11.1.4 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. 11.1.5 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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