LTC4311
6
4311fa
I
2
C and SMBus Overview
The I
2
C communication protocol employs open-drain
pull-down drivers with resistive or current source pull-
ups. This protocol allows multiple devices to drive and
monitor the bus without bus contention. The simplicity
of resistive or fi xed current source pull-ups is offset by
the slow rise times resulting when bus capacitance is
high. Rise times can be improved by using lower pull-up
resistor values or higher fi xed current source values, but
the additional current increases the low state bus voltage,
decreasing noise margins. Slow rise times can seriously
impact data reliability, enforcing a maximum practical bus
speed well below the established I
2
C or SMBus maximum
transmission rate.
The LTC4311 overcomes these limitations by providing a
boosted pull-up current only during positive bus transitions
to quickly slew large bus capacitances. Therefore, rise time
is dramatically improved, especially with maximum or out
of specifi cation I
2
C or SMBus loading conditions.
The LTC4311 has separate but identical circuitry for each
BUS output pin. The circuitry consists of a positive edge
slew rate detector and a voltage comparator. The voltage
comparator has a supply dependent threshold. At supply
voltages below 2.7V the comparator threshold is 0.3V
CC
,
and at higher voltages the comparator threshold is a
constant 0.8V. This allows the rise time accelerator to be
used in non-compliant systems where the bus thresholds
are optimized for low voltage operation, while still meet-
ing standard thresholds for compliant I
2
C and SMBus
systems.
The slew limited pull-up current is only turned on if the
bus line voltage is greater than the supply dependent
comparator threshold voltage and the positive slew rate
of the bus line is greater than the typical 0.2V/μs threshold
of the slew rate detector. The pull-up current remains on
until the voltage on the bus line is within 0.4V of V
CC
or
the slew rate drops below 0.2V/μs.
The pull-up current is slew limited to maintain signal
integrity for busses that have very little capacitive load.
In a lightly loaded system a strong pull-up could result in
fast edge rates that cause refl ections on the bus. These
refl ections can be detected by devices on the bus as extra
clock edges, could result in erroneous data, or cause a
stuck bus. An internal slew limit comparator limits the rate
the pull-up current can slew the bus lines to 100V/μs.
Auto Detect Standby Mode and Shutdown Mode
When BUS1 and BUS2 are both high the LTC4311 reduces
the standby supply current. Internal comparators detect
when the bus pins are within 400mV of V
CC
, and reduce
the supply current to 26μA. When the ENABLE pin is
grounded, the LTC4311 enters a low (<5μA) supply cur-
rent shutdown mode. Both bus pins are high impedance
in shutdown, regardless of the bus pin voltage.
OPERATION
