SLUSDQ4 April 2019 BQ79606A-Q1
The BQ79606A-Q1 has two analog ground pins termed AVSS pin 15 and 45. AVSS of pin 15 is a general-purpose analog ground associated with quiet grounds for sensitive internal analog circuitry and circuits supplied by VIO. The AVSS of pin 45 is used for the ADC internally, connect the decoupling capacitor of the REF1 to this pin for best ADC accuracy. The BQ79606A-Q1 device also has one CVSS pin for the daisy chain communication supply (CVDD). One DVSS pin is also present, supplying the ground for the internal digital core and supporting circuitry. In addition to these 4 ground pins, a power pad is located on the bottom of the device, and should be included in the GND plane to facilitate heat dissipation.
Creation of a good ground plane in the layout is crucial to getting optimal performance from the part. A good ground plane on a dedicated layer will improve measurement accuracy, reduce noise, and provide the necessary ESD, EMI, and EMC performance. There is a strong recommendation to have a minimum of four layers in the PCB, with one fully dedicated as an unbroken ground plane (except thermal reliefs). Avoid placing tracks on this layer to maintain the unbroken integrity of the plane structure.
All 4 device grounds, as well as the power pad, should connect to the ground plane with as short as possible track sections to minimize the effects of stray inductance on noise performance.
If more than one BQ79606A-Q1 is included on a single PCB assembly, each will require its own plane in the area surrounding the device. This is required because each device has its own VSS reference, often separated by more than 21V from VSS-to-VSS of adjacent ICs in the stack. These can exist on the same physical layer, with correct separation and clearance requirements.
Although the plane is employed as a solid GND reference with all grounds connected to it, good layout practice still requires locating any decoupling capacitors as close to the pin they are associated with as possible. This reduces inductance and keeps the loop area as small as possible, which in turn keeps the capacitors as effective as possible in reducing noise. In this document, the reference term for combined grounds connected to the ground plane is ground or GND.
The layout of BQ79606A-Q1 has 3 grounds: