LTC4308CDD-TRPBF datasheet(8/16 Pages) LINER

Part #	LTC4308CDD-TRPBF

Description	Low Voltage, Level Shifting Hot Swappable 2-Wire Bus Buffer with Stuck Bus Recovery

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Manufacturer	LINER [Linear Technology]

Direct Link	http://www.linear.com

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LTC4308

8

4308f

Start-Up

When the LTC4308 ﬁrst receives power on its VCC pin,

either during power-up or live insertion, it starts in an

under voltage lockout (UVLO) state, ignoring any activity

on the SDA or SCL pins until VCC rises above 2V (typical).

This ensures the LTC4308 does not try to function until

enough supply voltage is present.

During this time, the 1V precharge circuitry is actively

forcing 1V through 100k nominal resistors to the SDAOUT

and SCLOUT pins. Because SDAOUT and SCLOUT pins

may be plugged into a live backplane, where the voltage

on the backplane SDA and SCL busses can be anywhere

between 0V and VCC, precharging SCLOUT and SDAOUT

to 1V minimizes the worst-case voltage differential these

pins will see at the moment of contact, therefore minimizing

the amount of disturbance caused by the I/O card.

Once the LTC4308 exits from UVLO, it monitors both the

input and output pins for either a stop bit or a bus idle

condition to indicate the completion of data transactions.

When both sides are idle or one side has a stop bit while the

other is idle, the connection circuitry is activated, joining

the SDA and SCL pins on the input bus with those on the

output bus. Because SDAIN and SCLIN are monitored for

a stop bit or bus idle as a condition for connection, they

may also be used for Hot-Swapping, but note that these

pins are not precharged.

Connection Circuitry

Once the connection circuitry is activated, the functionality

of the input and output bus of the respective SDA or SCL

pins is identical. A low forced on either output or input pin

at any time results in both pin voltages forced low. The

LTC4308 SCLOUT and SDAOUT busses are tolerant of I2C

bus DC logic low voltages up to the VIL speciﬁcation of

0.3 • VCC, while the SCLIN and SDAIN busses are tolerant

of bus logic low voltages up to 0.6V. A high occurs when

all devices on the input and output pins release high.

When the LTC4308 senses a rising edge on either of the

output busses, with a slew rate greater than 0.8V/μs, the

internal pull-down device for the respective bus is deacti-

vated at bus voltages as low as 0.48V. This methodology

maximizes the effectiveness of the rise time accelerator

circuitry and maintains compatibility with other devices

in the LTC4300 bus buffer family. Care must be taken to

ensure devices participating in clock stretching or arbitra-

tion is capable of forcing logic low voltages below 0.48V

at the LTC4308’s SCLOUT and SDAOUT pins.

These important features ensure the I2C speciﬁcation

protocols such as clock stretching, clock synchroniza-

tion, arbitration, and acknowledge function seamlessly

in all cases as speciﬁed, regardless of how the devices in

the system are connected to the LTC4308.

Another key feature provided by the connection circuitry

is input and output bus capacitance isolation through

bidirectional buffering. Because of this isolation, the

waveforms on the input busses look slightly different than

the corresponding output bus waveforms, as described

in the next two sections.

Offset Voltages

When a logic low is driven on SDAIN or SCLIN, the LTC4308

regulates SDAOUT or SCLOUT, respectively, to a higher

voltage, typically 300mV above the driven low voltage.

When a logic low is driven on SCLOUT or SDAOUT, the

LTC4308 regulates SCLIN or SDAIN, respectively, to a volt-

age that is typically 200mV below the driven low voltage.

These offsets are nearly independent of pull-up current

(see Typical Performance Characteristics).

OPERATION

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LTC4308CDD-TRPBF Datasheet(PDF) 8 Page - Linear Technology

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