LTC6803-1/LTC6803-3
16
680313fa
OPERATION
watchdog event, but the CNFGO bit 7 will reflect the state
of this signal. Therefore, the WDTB pin can be used to
monitor external digital events if desired.
SERIAL PORT
Overview
The LTC6803 has an SPI bus compatible serial port. Several
devices can be daisy chained in series. There are two sets
of serial port pins, designated as low side and high side.
The low side and high side ports enable devices to be
daisy chained even when they operate at different power
supply potentials. In a typical configuration, the positive
power supply of the first, bottom device is connected
to the negative power supply of the second, top device,
as shown in Figure 1. When devices are stacked in this
manner, they can be daisy chained by connecting the high
side port of the bottom device to the low side port of the
top device. With this arrangement, the master writes to
or reads from the cascaded devices as if they formed one
long shift register. The LTC6803-1/LTC6803-3 translate the
voltage level of the signals between the low side and high
side ports to pass data up and down the battery stack.
Physical Layer
On the LTC6803-1/LTC6803-3, seven pins comprise the
low side and high side ports. The low side pins are CSBI,
SCKI, SDI and SDO. The high side pins are CSBO, SCKO
and SDOI. CSBI and SCKI are always inputs, driven by the
master or by the next lower device in a stack. CSBO and
SCKO are always outputs that can drive the next higher
device in a stack. SDI is a data input when writing to a
stack of devices. For devices not at the bottom of a stack,
SDI is a data output when reading from the stack. SDOI
is a data output when writing to and a data input when
reading from a stack of devices. SDO is an open-drain
output that is only used on the bottom device of a stack,
where it may be tied with SDI, if desired, to form a single,
bi-directional port. The SDO pin on the bottom device of
a stack requires a pull-up resistor. For devices up in the
stack, SDO should be tied to the local V
–
or left floating.
To communicate between daisy-chained devices, the high
side port pins of a lower device (CSBO, SCKO and SDOI)
should be connected through high voltage diodes to the
respective low side port pins of the next higher device
(CSBI, SCKI and SDI). In this configuration, the devices
communicate using current rather than voltage. To signal
a logic high from the lower device to the higher device,
the lower device sinks a smaller current from the higher
device pin. To signal a logic low, the lower device sinks
a larger current. Likewise, to signal a logic high from
the higher device to the lower device, the higher device
sources a larger current to the lower device pin. To signal
a logic low, the higher device sources a smaller current.
See Figure 2. Since CSBO, SCKO and SDOI voltages are
close to the V
–
of high side device, the V
–
of the high side
device must be at least 5V higher than that of the low side
device to guarantee current flows of the current mode
interface. It is recommended that high voltage diodes be
placed in series with the SPI daisy-chain signals as shown
if Figure 1. These diodes prevent reverse voltage stress
on the IC if a battery group bus bar is removed. See Bat-
tery Interconnection Integrity for additional information.
Standby current consumed in the current mode serial in-
terface is minimized when CSBI, SCKI and SDI are all high.
The voltage mode pin (V
MODE
) determines whether the low
side serial port is configured as voltage mode or current
mode. For the bottom device in a daisy-chain stack, this
pin must be pulled high (tied to V
REG
). The other devices
in the daisy chain must have this pin pulled low (tied to V
–
)
to designate current mode communication. To designate
the top-of-stack device for polling commands, the TOS
Figure 2. Current Mode Interface
+
–
WRITE
READ 1
V
SENSE
(WRITE)
+
–
V
SENSE
(READ)
680313 F02
HIGH SIDE PORT
ON LOWER DEVICE
LOW SIDE PORT
ON HIGHER DEVICE
