LTC4304

9

4304fa

A rising edge on ENABLE after a stuck bus condition has

occurred forces a connection between SDAIN, SDAOUT

and SCLIN, SCLOUT even if bus idle conditions are not

met. At this time the internal 30ms timer is reset but not

disabled.

Rise Time Accelerators

Once connection has been established, rise time accel-

erator circuits on all four SDA and SCL pins are activated

(assuming accelerators are enabled, see ACC pin descrip-

tion). These allow the use of a large pull-up resistor to

reduce power consumption, or bus capacitance beyond

requirements. During positive bus transitions, the LTC4304

switches in 3.5mA (typical) of current to quickly slew the

SDA and SCL lines once their DC voltages exceed 0.8V.

Choose a pull-up resistor so that the bus will rise on its

own at a rate of at least 0.8V/µs to guarantee activation

of the accelerators. Rise time accelerators turn off when

Rise time accelerators are automatically disabled during

automatic clocking.

Resistor Pull-Up Selection

The system pull-up resistors must be strong enough

to provide a positive slew rate of 0.8V/µs on the SDA

and SCL pins, in order to activate the rise time accelera-

tors during rising edges. Choose maximum resistor value

Rk

V

V

ns V

CpF

PULLUP MAX

BUS MIN

BUS

()

()

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–.

•

[ / ]

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0 8

1250

tive bus line.

resistor than 27.1k must be used, so 10k works fine.

Live Insertion and Capacitance Buffering Application

Figures 3 through 6 illustrate applications of the LTC4304

capacitance buffering features. In all of these applications,

note that if the I/O cards were plugged directly into the

backplane without the LTC4304 buffer, all of the backplane

and card capacitances would add directly together, making

rise- and fall-time requirements difficult to meet. Placing a

LTC4304 on the edge of each card, however, isolates the

card capacitance from the backplane. For a given I/O card,

the LTC4304 drives the capacitance on the card side and

the backplane must drive only the digital input capacitance

of the LTC4304, which is less than 10pF.

In most applications the LTC4304 will be used with a

pins. SDA and SCL are medium length pins to ensure that

the precharge circuitry to be activated on SDA and SCL

before they make contact. ENABLE is a short pin that is

pulled down when not connected. This is to ensure that the

connection between the backplane and the cards data and

clock busses is not enabled until the transients associated

with live insertion have settled.

CompactPCI power supply Hot Swap circuits. Use a pull-up

powering up after all the other pins have established con-

nection, the UVLO circuit ensures that the backplane and

the card data and clock busses are not connected until

the transients associated with live insertion have settled.

Owing to their small capacitance, the SDAIN and SCLIN

pins cause minimal disturbance on the backplane busses

when they make contact with the connector.

Hot Swap is a trademark of Linear Technology Corporation.

OPERATION

APPLICATIONS INFORMATION