Datasheet

LTC4300A-3
9
4300a3fa
Resistor Pull-Up Value Selection
The system pull-up resistors must be strong enough to
provide a positive slew rate of 1.25V/µs on the SDA and
SCL pins, in order to activate the boost pull-up currents
during rising edges. Choose maximum resistor value R
using the formula:
R ≤ (V
CC(MIN)
– 0.6)(800,000)/C
where R is the pull-up resistor value in ohms, V
CC(MIN)
is the minimum V
CC
voltage and C is the equivalent bus
capacitance in picofarads (pF).
In addition, regardless of the bus capacitance, always
choose R ≤ 16k for V
CC
= 5.5V maximum, R ≤ 24k for
V
CC
 = 3.6V maximum. The start-up circuitry requires
logic high voltages on SDAOUT and SCLOUT to connect
the backplane to the card, and these pull-up values are
needed to overcome the precharge voltage.
applicaTions inForMaTion
Figure 3. The LTC4300A-3 in a PCI Application Where All the Pins Have the Same Length.
ENABLE Should Be Held Low Until All Transients Associated with the Live Insertion Have Settled
Live Insertion and Capacitance Buffering Application
Figures 3 and 4 illustrate the usage of the LTC4300A-3
in applications 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, all of the backplane and card
capacitances would add directly together, making rise-
and fall time requirements difficult to meet. Placing a
LTC4300A-3 on the edge of each card, however, isolates
the card capacitance from the backplane. For a given I/O
card, the LTC4300A-3 drives the capacitance of every-
thing on the card and the backplane must drive only the
capacitance of the LTC4300A-3, which is less than 10pF.
V
CC
SDAIN
SCLIN
V
CC2
GND
SDAOUT
SCLOUT
ENABLE
4300a3 F03
R2
10k
R3
10k
R1
10k
R4
10k
R5
10k
R6
10k
I/O PERIPHERAL CARD 1
LTC4300A-3
C2 0.01µF
CARD_SCL
CARD_SDA
R7
10k
R8
10k
V
CC2
BACKPLANE
BACKPLANE
CONNECTOR
SDA
SCL
ENA2
ENA1
V
CC
C1
0.01µF
V
CC
SDAIN
SCLIN
V
CC2
GND
SDAOUT
SCLOUT
ENABLE
I/O PERIPHERAL CARD 2
LTC4300A-3
C4 0.01µF
CARD2_SCL
CARD2_SDA
C3
0.01µF