LTC4303CDD#TRPBF【PDF 8/12 页】IC BUS BUFFER I2C 8-DFN

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LTC4303

4303fb

Resistor Pull-Up Selection

The system pull-up resistors must be strong enough

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

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

tors during rising edges. Choose maximum resistor value

PULL-UP(MAX)

using the formula:

where V

BUSMIN

is the minimum operating pull-up sup-

ply voltage, and C

BUS

the total capacitance on respec-

tive bus line.

ns V

PULLUP MAX

BUS MIN

BUS

(

)

( )

[ ]

[ / ]

[ ]

& =

(

)

0 8

1250

For example, assume V

BUS

= V

= 3.3V, and assuming

?0% supply tolerance, V

BUSMIN

= 2.97V. With C

BUS

100pF, R

PULL-UP, MAX

= 27.1k. Therefore a smaller pull-up

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

Live Insertion and Capacitance Buffering Application

Figures 3 through 6 illustrate applications of the LTC4303

that take advantage of both its Hot Swap

controlling and

capacitance buffering features. In all of these applications,

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

backplane without the LTC4303 buffer, all of the backplane

and card capacitances would add directly together, making

rise- and fall-time requirements dif cult to meet. Placing a

LTC4303 on the edge of each card, however, isolates the

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

the LTC4303 drives the capacitance on the card side and

the backplane must drive only the digital input capacitance

of the LTC4303, which is less than 10pF.

In most applications the LTC4303 will be used with a

staggered connector where V

and GND will be long

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

the V

and GND pins make contact rst. This will allow

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 as-

sociated with live insertion have settled.

Figure 3 shows the LTC4303 in a CompactPCI

con gu-

ration. Connect V

and ENABLE to the output of one of

the CompactPCI power supply Hot Swap circuits. Use a

pull-up resistor to ENABLE for a card side enable/disable.

is monitored by a ltered UVLO circuit. With the V

voltage powering up after all the other pins have established

connection, 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.

Figure 4 shows the LTC4303 in a PCI application where all

of the pins have the same length. In this case, a RC lter

circuit on the I/O card with a product of 10ms provides

a lter to prevent the LTC4303 from becoming activated

until the transients associated with live insertion have

settled. Connect the capacitor between ENABLE and GND,

and the resistor from V

to ENABLE.

Hot Swap is a trademark of Linear Technology Corporation.

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