LTC4311

6

4311fa

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 fixed 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 fixed 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

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

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

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-

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

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 reflections on the bus. These

reflections 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

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