Datasheet

TSDA
TSCL
TMDS141
RSDA
RSCL
EN
TSDA
TSCL
TMDS141
RSDA
RSCL
EN
SDA
SCL
BUS
MASTER
SDA
SCL
BUS
SLAVE
TSDA
TSCL
TMDS141
RSDA
RSCL
EN
3.3V
3.3V
3.3V
Source Sink
Repeater
Rup
SOURCE
Rup
2
Rup
1
C
1
C
2
C
3
C
3
C
1
5V
5V
C
2
5V
Rup
SINK
C
2
5V
Rup
SINK
C
2
Rup
SOURCE
R
up(min)
+ V
DD
ńlsink
T + k RC
V(t) + V
DD
(1 * e
*tńRC
)
TMDS141
SLLS737D –JUNE 2006–REVISED SEPTEMBER 2011 www.ti.com
Figure 24. Typical Series Application
I
2
C Pull-up Resistors
The pull-up resistor value is determined by two requirements:
1. The maximum sink current of the I
2
C buffer:
The maximum sink current is 3 mA or slightly higher for an I
2
C driver supporting standard-mode I
2
C
operation.
(1)
2. The maximum transition time on the bus:
The maximum transition time, T, of an I
2
C bus is set by an RC time constant, where R is the pull-up resistor
value, and C is the total load capacitance. The parameter, k, can be calculated from equation 3 by solving for
t, the times at which certain voltage thresholds are reached. Different input threshold combinations introduce
different values of t. Table 1 summarizes the possible values of k under different threshold combinations.
(2)
(3)
Table 1. Value k Upon Different Input Threshold Voltages
V
th-
\V
th+
0.7V
DD
0.65V
DD
0.6V
DD
0.55V
DD
0.5V
DD
0.45V
DD
0.4V
DD
0.35V
DD
0.3V
DD
0.1V
DD
1.0986 0.9445 0.8109 0.6931 0.5878 0.4925 0.4055 0.3254 0.2513
0.15V
DD
1.0415 0.8873 0.7538 0.6360 0.5306 0.4353 0.3483 0.2683 0.1942
0.2V
DD
0.9808 0.8267 0.6931 0.5754 0.4700 0.3747 0.2877 0.2076 0.1335
0.25V
DD
0.9163 0.7621 0.6286 0.5108 0.4055 0.3102 0.2231 0.1431 0.0690
0.3V
DD
0.8473 0.6931 0.5596 0.4418 0.3365 0.2412 0.1542 0.0741 -
From equation 1, R
up(min)
= 5.5V/3mA = 1.83 kΩ to operate the bus under a 5-V pull-up voltage and provide less
than 3 mA when the I
2
C device is driving the bus to a low state. If a higher sink current, for example 4 mA, is
allowed, R
up(min)
can be as low as 1.375 kΩ.
Given a 5-V I
2
C device with input low and high threshold voltages at 0.3 V
dd
and 0.7 V
dd
, the valued of k is
0.8473 from Table 1. Taking into account the 1.83-kΩ pull-up resistor, the maximum total load capacitance is
C
(total-5V)
= 645 pF. C
cable(max)
should be restricted to be less than 545 pF if C
source
and C
i
can be as heavy as 50
pF. Here the C
i
is treated as C
sink
, the load capacitance of a sink device.
Fixing the maximum transition time from Table 1, T = 1 μs, and using the k values from Table 1, the
recommended maximum total resistance of the pull-up resistors on an I
2
C bus can be calculated for different
system setups.
To support the maximum load capacitance specified in the HDMI spec, C
cable(max)
= 700pF/C
source
= 50pF/C
i
=
50pF, R
(max)
can be calculated as shown in Table 2.
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