LTC4303

6

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Start-Up

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

voltage lockout (UVLO) state, ignoring any activity on the

During this time, the precharge circuitry is active and

forces 1V through 200k nominal resistors to the SDA

and SCL pins. Because the I/O card is being plugged

into a live backplane, the voltage on the backplane SDA

Precharging the SCL and SDA pins to 1V minimizes the

worst-case voltage differential these pins will see at the

moment of connection, therefore minimizing the amount

of disturbance caused by the I/O card.

Once the LTC4303 comes out of UVLO, it assumes that

SDAIN and SCLIN have been inserted into a live system

and that SDAOUT and SCLOUT are being powered up at

the same time as itself. Therefore, it looks for either a stop

bit or bus idle condition on the input side to indicate the

completion of a data transaction. When either one occurs,

the part also verifies that both the SDAOUT and SCLOUT

voltages are high. When all of these conditions are met,

the input-to-output connection circuitry is activated, join-

ing the SDA and SCL busses on the I/O card with those

on the backplane and READY goes high.

Connection Circuitry

Once the connection circuitry is activated, the functionality

of the SDAIN and SDAOUT pins is identical. A low forced

on either pin at any time results in both pin voltages be-

ing low. For proper operation, logic low input voltages

should be no higher than 0.4V with respect to the ground

pin voltage of the LTC4303. SDAIN and SDAOUT enter

a logic high state only when all devices on both SDAIN

and SDAOUT release high. The same is true for SCLIN

and SCLOUT. This important feature ensures that clock

stretching, clock synchronization, arbitration and the ac-

knowledge protocol always work, regardless of how the

devices in the system are tied to the LTC4303.

Another key feature of the connection circuitry is that it

provides bidirectional buffering, keeping the backplane

and card capacitances isolated. Because of this isolation,

OPERATIO

the waveforms on the backplane busses look slightly

different than the corresponding card bus waveforms, as

described here.

Input to Output Offset Voltage

LTC4303’s data or clock pins, the LTC4303 regulates the

to a slightly higher voltage, as directed by the following

equation:

where R is the bus pull-up resistance in ohms. For ex-

ample, if a device is forcing SDAOUT to 10mV where

then the voltage on SDAIN = 10mV + 75mV + (3.3/10000)

• 20 = 91.6mV (typical). See the Typical Performance

Characteristics section for curves showing the offset

Bus Stuck Low Time-Out

When SDAOUT or SCLOUT is low, an internal timer starts.

The timer is only reset when SDAOUT and SCLOUT are

both high. If they do not go high within 30ms (typical),

the connection between SDAIN and SDAOUT, and SCLIN

and SCLOUT is broken. After a delay of at least 40µs the

LTC4303 automatically generates up to 16 clock pulses at

8.5kHz (typical) on SCLOUT in an attempt to unstick the

bus. When SDAOUT and SCLOUT go high, reconnection

occurs when the conditions described in the “Start-Up”

section above are satisfied.

When powering up into a bus stuck low condition, the

connection circuitry joining the SDA and SCL busses on

the I/O card with those on the backplane is not activated.

30ms after UVLO, automatic clocking takes place as

described above.

Propagation Delays

During a rising edge, the rise-time on each side is de-

termined by the bus pull-up resistor and the equivalent

capacitance on the line. If the pull-up resistors are the

same, a difference in rise-time occurs which is directly

proportional to the difference in capacitance between