LTC4308
11
4308f
APPLICATIONS INFORMATION
Resistor Pull-Up Value Selection
To guarantee the SDAOUT and SCLOUT rise time accelera-
tors are activated during a rising edge, the bus must rise
on its own with a positive slew rate of at least 0.8V/μs. To
achieve this, choose a maximum resistor value R
PULLUP
using the formula:
R
PULLUP
≤
(V
BUS(MIN)
− 0.8V)•1250ns / V
C
BUS
Where R
PULLUP
is the pull-up resistor value in kΩ, V
BUS(MIN)
is the minimum bus pull-up supply voltage and C
BUS
is
the equivalent bus capacitance in pF.
To estimate the value of C
BUS
, use a general rule of 20pF
of capacitance per device on the bus (10pF for the device
and 10pF for interconnect).
In addition, R
PULLUP
must be strong enough to overcome
the precharge voltage and provide logic highs on SDAOUT
and SCLOUT for the start-up and connection circuitry to
connect the backplane to the card. To meet this require-
ment, always choose
R
PULLUP
≤ 75k
V
BUS(MIN)
− V
THR(MAX)
V
THR(MAX)
− 1V
where V
THR(MAX)
is the maximum specifi ed Logic Input
Threshold Voltage, V
THR
.
Further, on SDAIN and SCLIN and for heavily loaded
systems on SDAOUT and SCLOUT, where the selected
R
PULLUP
value causes the bus to rise at a rate slower than
0.8V/μs, users must also guarantee
R
PULLUP
≤
V
BUS(MIN)
− V
THR(MAX)
100μA
Live Insertion and Capacitance Buffering Application
Figure 4 and 5 illustrate applications of the LTC4308 that
take advantage of the LTC4308’s Hot Swap™, capacitance
buffering and output pin precharge features. If the I/O
cards were plugged directly into the backplane without the
LTC4308 buffer, all of the backplane and card capacitances
would add directly together, making rise time and fall time
requirements diffi cult to meet. Placing an LTC4308 on the
edge of each card isolates the card capacitance from the
backplane. For a given I/O card, the LTC4308 drives the
capacitance of everything on the card and the backplane
must drive only the capacitance of the LTC4308, which
is less than 10pF.
Figure 4 shows the LTC4308 used in the typical staggered
connector application, where V
CC
and GND are the longest
“early power” pins. The “early power” pins ensure the
LTC4308 is initially powered and forcing the 1V precharge
voltage on the medium length SDA and SCL output pins
before they contact with the backplane busses. Coupled
with ENABLE as the shortest pin, passively pulled to ground
by a resistor, the staggered approach provides additional
time for transients associated with live insertion to settle
before the LTC4308 can be enabled.
Figure 5 shows the LTC4308 in an application where all
of the pins have the same length. In this application, a
resistor is used to hold the ENABLE pin low during live
insertion, until the backplane control circuitry can enable
the device.
Level Shifting Applications
Systems requiring different supply voltages for the
backplane side and the card side can use the LTC4308
for bidirectional level shifting, as shown in Figures 4, 5,
and 7. The LTC4308 can level shift between bus pull-up
supplies as low as 0.9V to as high as 5.5V. Level shifting
allows newer designs that require lower voltage supplies,
such as EEPROMs and microcontrollers, the capability to
interface with legacy backplanes which may be operating
at higher supply voltages.
The LTC4308’s negative offset voltage from output to
input allow level shifting applications with high SDAOUT
and SCLOUT V
OL
to effectively translate to the low voltage
SDAIN and SCLIN busses. Figure 7 shows an application
where 200Ω resistors, used to provide additional ESD
protection for the Temperature Sensor’s internal low
impedance pull-down device, generate high V
OL
on the
SDAOUT and SCLOUT busses.
