Datasheet LTC4308 (Analog Devices) - 10

LTC4308
OPERATION
When powering up into a bus stuck low condition, the
Rise Time Accelerators
connection circuitry connecting the SDA and SCL pins are Once connection has been established, rise time accelera- not activated. 30ms after UVLO, automatic clocking and tor circuits on SDAOUT and SCLOUT are enabled. During stop bit generation takes place as described above. positive bus transitions of at least 0.8V/μs, the rise time accelerators provide strong, slew-limited pull-up currents
READY Digital Output
to force the bus voltage to rise at a rate of 100V/μs. This pin provides a digital fl ag which is low when either The rise time accelerators signifi cantly improve reli- ENABLE is low, the start-up sequence described earlier ability and performance in I2C systems in several ways. in this section has not been completed, or the LTC4308 First, due to the accelerator’s signifi cantly lower pull-up has disconnected the input and output busses due to a impedance, as compared to the bus pull-up resistance, bus stuck low condition. READY goes high when ENABLE the system is less susceptible to noise on rising edges, is high and start-up is complete. The pin is driven by an providing smooth, controlled transitions for both small open-drain pull-down device capable of sinking 3mA while and large systems. Second, the accelerators allow users holding 0.4V on the pin. Connect a resistor to the bus to choose larger bus pull-up resistors, reducing power pull-up supply to provide the pull-up. consumption and improving logic low noise margins or
ENABLE
to design with bus capacitances beyond those specifi ed in the I2C specifi cations. When the ENABLE pin is driven below 0.45V with respect to the LTC4308’s ground, the input pin is disconnected For these reasons, it is strongly recommended that users from the output pin and the READY pin is pulled low. choose bus pull-up resistors that guarantee the output When the pin is driven above 0.75V, the part waits for busses will rise on their own at a rate of at least 0.8 V/μs to data transactions on both the input and output pins to be ensure activation of the accelerators. See the Applications complete (as described in the Start-Up section) before Information section for selecting pull-up resistor sizes. connecting the two pins. At this time the internal pull-down It is important to connect SDAOUT and SCLOUT pins to on READY releases. a bus whose pull-up supply is equal to or greater than A rising edge on ENABLE after a bus stuck low condition the LTC4308’s supply to ensure the accelerators do not has occurred forces a connection between SDAIN, SDAOUT, source current through the pull up resistors into the pull- and SCLIN, SCLOUT even if the bus stuck low condition up supply. has not been cleared. At this time the 30ms timer is reset, The rise time accelerators are internally disabled until the but not disabled. sequence of events described in the start-up section has been completed, as well as during automatic clocking and stop bit generation for a bus stuck low recovery event. 4308f 10