Datasheet

LTC4311
7
4311fa
Selecting the values of R
S
and R
P
The typical confi guration for the data bus for a 2-wire bus
is shown in Figure 1. The parameters R
P
and R
S
should be
chosen carefully. A description of the process for choosing
the values of R
P
and R
S
follows.
An external pull-up resistor R
P
is required in each bus
line to supply a steady state pull-up current if the bus is
at logic zero. This pull-up current is used for slewing the
bus line during the initial portion of the positive transition
in order to activate the LTC4311 pull-up current.
Using an external pull-up resistor R
P
to supply steady
state pull-up current provides the freedom to adjust rise
time versus fall time as well as defi ning the low state
logic-level (V
OL
).
For I/O stage protection from ESD and high voltage spikes
on the bus, a series resistor R
S
(Figure 1) is sometimes
added to the open drain driver of the bus agents.
R
S
V
CC
C
BUS
Bus
R
ON
4311 F01
DATA
IN
LTC4311
DYNAMIC
CURRENT
PULL-UP
DATA
OUT
R
P
Figure 1. Typical 2-Wire Bus Confi guration
APPLICATIONS INFORMATION
Low State Noise Margin
A low value of V
OL
, the low state logic level, is desired
for good noise margin. V
OL
is calculated as follows:
V
OL
=
R
L
• V
CC
R
L
+R
P
(1)
R
L
is the series sum of R
S
and R
ON
, the on resistance of
the open-drain driver.
Increasing the value of R
P
decreases the value of V
OL
.
Increasing R
L
increases the value of V
OL
.
Initial Slew Rate
The initial slew rate, SR, of the bus is determined by:
SR =
V
CC
–V
OL
R
P
•C
BUS
(2)
SR must be greater than SR
THRESH
, the LTC4311 slew rate
detector threshold (0.5V/μs max), in order to activate the
pull-up current.
I
2
C Rise and Fall Time
Rise time of an I
2
C line is derived using equation 3.
t
r
= – R
p
•C
BUS
•
ln
V
IHMIN
–V
CC
–R
p
•I
PULLUPAC
V
ILMAX
–V
CC
–R
p
•I
PULLUPAC
⎧
⎨
⎪
⎩
⎪
⎫
⎬
⎪
⎭
⎪
(3)
Fall time of an I
2
C line is derived using equation 4.
t
f
= R
T
•C
BUS
•ln
V
IHMIN
V
CC
•(R
P
+R
L
)–R
L
V
ILMAX
V
CC
•(R
P
+R
L
)–R
L
⎧
⎨
⎪
⎪
⎩
⎪
⎪
⎫
⎬
⎪
⎪
⎭
⎪
⎪
(4)
where R
T
is the parallel equivalent of R
P
and R
L
.
Both the values of R
P
and R
S
must be chosen carefully
to meet the low state noise margin and all bus timing
requirements.
A discussion of the electrical parameters affected by the
values of R
S
and R
P
, as well as the general procedure for
selecting the values of R
S
and R
P
follows.