LTC4311
8
4311fa
APPLICATIONS INFORMATION
A general procedure for selecting R
P
and R
L
is as fol-
lows:
1. R
L
is fi rst selected based on the I/O protection
requirement. Generally, an R
S
of 100Ω is suffi cient for
high voltage spikes and ESD protection. R
ON
is
determined by the size of the open-drain driver, a large
driver will have a lower R
ON
.
2. The value of R
P
is determined based on the V
OL
and
minimum slew rate requirements. The V
OL
will determine
the smallest resistance value that can be used in a
system, and the minimum slew requirement will bound
the resistance on the upper end. Generally the largest
value of resistance that meets the minimum slew rate
with some margin will be selected.
3. For I
2
C systems incorporating the LTC4311, the rise
times are met under most loading conditions, due to
the strong accelerator current. The pull-down drivers
are typically low impedance, and therefore fall times
are not generally an issue. Rise and fall time
requirements must be verifi ed using equations 3 and
4 (for an I
2
C system) or equations 5 to 8 (for an SMBus
system). The value chosen for R
P
must ensure that
both the rise and fall time specifi cations are met
simultaneously.
I
2
C Design Example
Given the following conditions and requirements:
V
CC
= 3.3V NOMINAL
V
OL
= 0.4V MAXIMUM
C
BUS
= 600pF
V
ILMAX
= 0.99V,V
IHMIN
= 2.31V
t
r
= 0.3µs MAXIMUM,t
f
= 0.3µs MAXIMUM
(9)
If an R
S
of 100Ω is used and the max R
ON
of the driver is
200Ω, then R
L
= 200Ω + 100Ω = 300Ω. Use equation 1
to fi nd the required R
P
to meet V
OL
.
R
P
=
300Ω•(3.3V – 0.4V)
0.4V
R
P
= 2.175k
(10)
For an I
2
C system with fi xed input levels, V
ILMAX
= 1.5V
and V
IHMIN
= 3V. For I
2
C systems with V
CC
related input
levels, V
ILMAX
= 0.3V
CC
and V
IHMIN
= 0.7V
CC
.
C
BUS
is the total capacitance of the I
2
C line.
SMBus Rise and Fall Time
Rise time of a SMBus line is derived using equations 5,
6 and 7.
t
r
= t
1
+t
2
(5)
t
1
is the time from when the bus crosses the lower slew
rate measurement point, until the bus reaches V
THR
and the
accelerators fi re. The time from when the accelerators fi re
until the bus reaches the upper slew rate measure point is
given by t
2
. Equations for t
1
and t
2
are given here:
t
1
= –R
P
•C
BUS
•ln
V
THR
–V
CC
V
ILMAX
– 0.15V – V
CC
⎧
⎨
⎩
⎫
⎬
⎭
(6)
If (V
ILMAX
– 0.15V) > V
THR
, then t
1
= 0
t
2
= –R
P
•C
BUS
•
ln
V
IHMIN
+ 0.15V – V
CC
–R
P
•I
PULLUPAC
V
THR
–V
CC
–R
P
•I
PULLUPAC
⎧
⎨
⎩
⎫
⎬
⎭
(7)
Fall time of an SMBus line is derived using equation 8:
t
f
= R
T
•C
BUS
•
ln
V
IHMIN
+ 0.15V
V
CC
•(R
P
+R
L
)–R
L
V
ILMAX
– 0.15V
V
CC
•(R
P
+R
L
)–R
L
⎧
⎨
⎪
⎪
⎩
⎪
⎪
⎫
⎬
⎪
⎪
⎭
⎪
⎪
(8)
For an SMBus system, V
ILMAX
= 0.8V and V
IHMIN
= 2.1V.
C
BUS
is the total bus capacitance of the SMBus line.
