10.1 Layout Guidelines

For typical applications, the following guidelines and practices should be followed closely:

• VREF and AGND pins require a high-quality 10-µF capacitor be connected between them, in very close physical proximity to the device pins, using short track lengths to minimize the effects of track inductance on signal quality.
  • The AGND pin should be connected to VSS. Device VSS connections should be brought to a single point close to the IC to minimize layout-induced errors. The device tab should also be connected to this point, and is a convenient common VSS location. The internal VREF should not be used externally to the device by user circuits.
• The internal analog supply should be bypassed at the LDOA pin with a good-quality, low-ESR, 2.2-µF ceramic capacitor.

NOTE

Because the LDODx inputs are pulled to approximately 7 V during programming, programming time MUST be < 50 ms.

• The bq76PL536A has a low-dropout (LDO) regulator provided to power the thermistors and other external circuitry. The input for this regulator is VBAT. The output of REG50 is typically 5 V. A minimum 2.2-µF capacitor is required for stable operation. The output is internally current-limited and is reduced to near zero, if excess current is drawn, causing die temperatures to rise to unacceptable levels. The 2.2-µF output capacitor is required whether REG50 is used in the design or not. REG50 is disabled in SLEEP mode, may be turned off under thermal-shutdown conditions, and therefore should not be used as a pull-up source for terminating device pins where required.
• The bq76PL536A includes a general-purpose input/output pin controlled by the IO_CONTROL[GPIO_OUT] bit. The state of this bit is reflected on the pin. To use the pin as an input, program GPIO_OUT to a 1, and then read the IO_CONTROL[GPIO_IN] bit. A pull-up (10 kΩ–1 MΩ, typical) is required on this pin if used as an input. If the pull-up is not included in the design, system firmware must program a 0 in IO_CONTROL[GPIO_OUT] to prevent excess current draw from the floating input. Use of a pull-up is recommended in all designs to prevent an unintentional increase in current draw.
• Device-to-device (D2D) communications makes use of a unique, current-mode interface which provides common-mode voltage isolation between successive bq76PL536As. This vertical bus (VBUS) is found on the _N and corresponding _S pins. It provides high-speed I/O for both the SPI bus and the direct I/O pins CONV and DRDY. The current-mode interface minimizes the effects of wiring capacitance on the interface speed. The _S (south-facing) pins connect to the next-lower device (operating at a lower potential) in the stack of bq76PL536As. The _N (North facing) pins connect to the next-higher device. The pins cannot be swapped; _S always points South, and _N always point North. The _S and _N pins are interconnected to the pin with the same name, but opposite suffix.
  • All pins operate within the voltages present at the BAT and VSS pins.
  • The maximum SCLK frequency is limited by the number of devices in the vertical stack and other factors. Each device imposes an approximately 30-ns delay on the round trip communications speed; that is, from SCLK rise time (an input to all devices) to the SDO pin transition time requires approximately 30 ns per device. The designer must add to this the delay caused by the PCB trace (in turn determined by the material and layout), any connectors in series with the connection, and any other wiring or cabling between devices in the system.
• When designing the layout, several considerations need to be taken into account.
  • First, in a stacked system, individual ground planes are necessary for proper noise rejection and stability of the circuits.
  • Second, the ground (VSS) reference per circuit block is unique. The most negative connection, per block “CELL0”, is the ground (VSS) reference for each IC. Do not connect ground references from different ICs. Only the ground reference CELL0, of the most southerly IC, is safe to connect non-isolated test equipment grounds.

CAUTION

Be careful as the BAT and VSS pins may be several hundred volts above system ground, depending on their position in the stack.

NOTE

North (_N) pins of the top, most-positive device in the stack, should be connected to the BAT1(2) pins of the device for correct operation of the string. South (_S) pins of the lowest, most-negative device in the stack, should be connected to VSS of the device.

The PowerPAD™ package is a thermally enhanced standard-size IC package designed to eliminate the use of bulky heat sinks and slugs traditionally used in thermal packages. This package can be easily mounted using standard printed circuit board (PCB) assembly techniques, and can be removed and replaced using standard repair procedures. See Figure 68.

The PowerPAD™ package is designed so that the lead frame die pad (or thermal pad) is exposed on the bottom of the IC. This provides an extremely low-thermal resistance (R_θJC) path between the die and the exterior of the package. The thermal pad on the bottom of the IC can then be soldered directly to the printed circuit board (PCB), using the PCB as a heat sink. In addition, through the use of thermal bias, the thermal pad can be directly connected to a ground plane or special heat sink structure designed into the PCB.

Figure 68. Section View of PowerPAD™ Package and Top View of Solder Mask and PAD

10.2 Layout Example

Figure 69. Layout Schematic