The LTC4305 is a 2-channel 2-wire bus multiplexer/
switch with bus buffers to provide capacitive isolation
between the upstream bus and downstream buses. Mas-
ters on the upstream 2-wire bus (SDAIN and SCLIN) can
command the LTC4305 to neither, either or both of the 2
downstream buses. Masters can also program the LTC4305
to disconnect the upstream bus from the downstream
buses if the bus is stuck low.
Undervoltage Lockout (UVLO) and ENABLE
The LTC4305 contains undervoltage lockout circuitry that
maintains all of its SDA, SCL and ALERT pins in high
impedance states until the device has sufficient V
ply voltage to function properly. It also ignores any
attempts to communicate with it via the 2-wire buses in
this condition. When the ENABLE pin voltage is low (below
0.8V), all control bits are reset to their default high
impedance states, and the LTC4305 ignores 2-wire bus
commands. However, with ENABLE low, the LTC4305 still
monitors the ALERT1–ALERT2 pin voltages and pulls the
ALERT pin low if any of ALERT1–ALERT2 is low. When
ENABLE is high, devices can read from and write to the
Connection Circuitry
Masters on the upstream SDAIN/SCLIN bus can write to
the Bus 1 FET State and Bus 2 FET State bits of register 3
to connect to any combination of downstream channels.
By default, the Connection Circuitry shown in the block
diagram will only connect to downstream channels whose
corresponding Bus Logic State bits in register 3 are high
at the moment that it receives the connection command.
If the LTC4305 is commanded to connect to multiple
channels at once, it will only connect to the channels that
are high. This prevents the master on the upstream bus
from connnecting to a downstream channel that may be
stuck low. Masters can override this feature by setting the
Connection Requirement Bit of register 2 high. With this
bit high, the LTC4305 executes connection commands
without regard to the logic states of the downstream
Upon receiving the connection command, the Connection
Circuitry shown in the block diagram will activate the
Upstream-Downstream Buffers under two conditions:
first, the master must be commanding connection to one
or more downstream channels, and second, there must be
no stuck low condition (see “Stuck Low Timeout Fault”
discussion that follows). If the connection command is
successful, the Upstream-Downstream Buffer circuitry
passes signals between the upstream bus and the con-
nected downstream buses. The LTC4305 also turns off its
N-channel MOSFET open-drain pull-down on the READY
pin, so that READY can be pulled high by its external pull-
up resistor.
Upstream-Downstream Buffers
Once the Upstream-Downstream Buffers are activated,
the functionality of the SDAIN and any connected down-
stream SDA pins is identical. A low forced on any con-
nected SDA pin at any time results in all pins being low.
External devices must pull the pin voltages below 0.4V
worst-case with respect to the LTC4305’s ground pin to
ensure proper operation. The SDA pins enter a logic high
state only when all devices on all connected SDA pins
force a high. The same is true for SCLIN and the connected
downstream SCL pins. This important feature ensures
that clock stretching, clock arbitration and the acknowl-
edge protocol always work, regardless of the how the
devices in the system are connected to the LTC4305.
The Upstream-Downstream Buffers provide capacitive
isolation between SDAIN/SCLIN and the downstream
connected buses. Note that there is no capacitive isolation
between connected downstream buses; they are only