Datasheet

bq76PL536-Q1
SLUSAB1 –MAY 2011 www.ti.com
The pins of the base IC (only) in a stack should have the SCLK_H and SDI_H pins terminated with pullups to
minimize current draw of the part if the host ever enters a state where the pins are not driven, i.e., held in the
high-impedance state by the host. In non-base devices, the _H pins are forced to be all outputs driven low when
the HSEL pin is high. In non-base devices, all _H pins should remain unconnected.
The CS_H has a pullup resistor of approximately 100 kΩ. SDO_H is a 3-state output and is terminated with a
weak pullup.
Designer Note: When V
BAT
is at or below the UVLO trip point voltage, the internal LDO which supplies the
xxxx_H host SPI communications pins (VLODx) begins to fall out of regulation. The output high voltage on
the xxxx_H pins falls off with the LDO voltage in an approximately linear manner until at the POR voltage trip
point it is reduced to approximately 3.5 V. This action is not tested in production.
Application Notes on the Host SPI Interface Pin States
The CS_H pin is active-low. The host asserts the pin to a logic zero to initiate communications. The CS pin
should remain low until the end of the current packet. When the CS_H pin is asserted, the SPI receiver and
interface of the device are reset and resynchronized. This action ensures that a slave device that has lost
synchronization during a previous transmission or as the result of noise on the bus does not remain permanently
hung. CS_H must be driven false (high) between packets; see AC Timing Characteristics for timing details.
Device-to-Device Vertical Bus (VBUS) Interface
Device-to-device (D2D) communications makes use of a unique, current-mode interface which provides
common-mode voltage isolation between successive bq76PL536-Q1s. 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
bq76PL536-Q1s. 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. Use
caution; these pins may be several hundred volts above system ground, depending on their position in
the stack.
Designer 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 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, i.e., from SCLK rising (an
input to all devices) to the SDO pin transitioning requires ~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. To maximize speed, these other
system components should be carefully selected to minimize delays and other detrimental effects on signal
quality. Wiring and connectors should receive special attention to their transmission line characteristics.
Other factors which should be considered are clock duty cycle, clock jitter, temperature effects on clock and
system components, user-selected drive level for the level-shift interface, and desired design margin.
The VBUS SPI interface is placed in a low-power mode when CS_H is not asserted on the base device.
The CS_N/S pins are asserted by a logic high on the vertical interface bus (logically inverted from CS_H). This
creates a default VBUS CS condition of logic low, reducing current consumption to a minimum.
To reduce power consumption of the SPI interface to a minimum, the SCLK_H and SDI_H should be maintained
at a logic low (de-asserted) while CS_H is asserted (low). Most SPI buses are operated this way by
microcontrollers. The VBUS versions of these signals are not inverted from the host interface. The device also
de-asserts by default the SDO_N/S pins to minimize power consumption.
28 Copyright © 2011, Texas Instruments Incorporated