Datasheet LTC4311 (Analog Devices) - 6

LTC4311
OPERATION I2C and SMBus Overview
are optimized for low voltage operation, while still meet- ing standard thresholds for compliant I2C and SMBus The I2C communication protocol employs open-drain systems. pull-down drivers with resistive or current source pull- ups. This protocol allows multiple devices to drive and The slew limited pull-up current is only turned on if the monitor the bus without bus contention. The simplicity bus line voltage is greater than the supply dependent of resistive or fi xed current source pull-ups is offset by comparator threshold voltage and the positive slew rate the slow rise times resulting when bus capacitance is of the bus line is greater than the typical 0.2V/μs threshold high. Rise times can be improved by using lower pull-up of the slew rate detector. The pull-up current remains on resistor values or higher fi xed current source values, but until the voltage on the bus line is within 0.4V of VCC or the additional current increases the low state bus voltage, the slew rate drops below 0.2V/μs. decreasing noise margins. Slow rise times can seriously The pull-up current is slew limited to maintain signal impact data reliability, enforcing a maximum practical bus integrity for busses that have very little capacitive load. speed well below the established I2C or SMBus maximum In a lightly loaded system a strong pull-up could result in transmission rate. fast edge rates that cause refl ections on the bus. These The LTC4311 overcomes these limitations by providing a refl ections can be detected by devices on the bus as extra boosted pull-up current only during positive bus transitions clock edges, could result in erroneous data, or cause a to quickly slew large bus capacitances. Therefore, rise time stuck bus. An internal slew limit comparator limits the rate is dramatically improved, especially with maximum or out the pull-up current can slew the bus lines to 100V/μs. of specifi cation I2C or SMBus loading conditions.
Auto Detect Standby Mode and Shutdown Mode
The LTC4311 has separate but identical circuitry for each BUS output pin. The circuitry consists of a positive edge When BUS1 and BUS2 are both high the LTC4311 reduces slew rate detector and a voltage comparator. The voltage the standby supply current. Internal comparators detect comparator has a supply dependent threshold. At supply when the bus pins are within 400mV of VCC, and reduce voltages below 2.7V the comparator threshold is 0.3V the supply current to 26μA. When the ENABLE pin is CC, and at higher voltages the comparator threshold is a grounded, the LTC4311 enters a low (<5μA) supply cur- constant 0.8V. This allows the rise time accelerator to be rent shutdown mode. Both bus pins are high impedance used in non-compliant systems where the bus thresholds in shutdown, regardless of the bus pin voltage. 4311fa 6