Datasheet

R
upSource
V
DDSource
Source Sink
Ron
SCL_SINK
SDA_SINK
SCL
SDA
Driver(Source)
I
to-Sink
I
to-Source
V
DDSink
R
upSink
V
CCRx
R
upRx
Driver(Sink)
Ito * Sink +
V
dd
* V
ol(Sink)max
R
upSource
ø R
upSink
lto * Sink +
V
dd
R
upSource
ø R
upSink
V
ith(Source)min
u lto * Sink Ron ) V
ol(Sink)max
R
upSink
w
V
dd
Ron R
upSource
(V
ith(Source)min
* V
ol(Sink)max
) R
upSource
* V
dd
Ron
R
upRx
w
V
ccRx
Ron
(V
ith(Sink)min
* V
ol(Source)max
)
TMDS351
www.ti.com SLLS840B –MAY 2007– REVISED JULY 2011
HPD Pins
The HPD circuits are powered by the 5-V supply. They provide 5-V TTL output signals to the SOURCE with a
typical 1-kΩ output resistance. An external 1-kΩ resistor is not needed here. The HPD output of the selected
source port follows the logic level of the HPD_SINK input. Unselected HPD outputs are kept low. When the
device is in standby mode, all HPD outputs follow HPD_SINK. A 1-kΩ resistor to ground keeps all HPD outputs
low in standby mode if a fixed low state is preferred.
DDC Channels
The DDC circuits (SDA, SCL) are powered by a 5-V supply. The I/O pins can connect to the 5-V termination
voltages directly. A 47-kΩ pull-up resistor to the 5 V is recommended on the SCL1, SCL2, and SCL3 pins. There
is no pull-up resistor on the SDA pins. The pull-up resistor can be replaced with a different value.
Figure 17. Simplified Electrical Circuit Model for DDC Channel
In Figure 17, when the Driver (Sink) pulls the bus low, the highest voltage level is V
ol(Sink)max
. The current flow
through the pass-gate resistor can be presented as:
(1)
where the V
ddsource
= V
ddsink
= V
dd
To simplify the equation, V
ol(Sink)max
is set equal to 0 V to reach equation (2):
(2)
The voltage at the input of the SINK is Ito - Sink × Ron + V
ol(Sink)max
, which should be lower than the minimum
input low threshold voltage of the Driver (Source), V
ith(Source)min
to keep the bus in correct interoperations.
(3)
By combining equations (2) and (3), the minimum pull-up resistor at the Sink input is:
(4)
Applying the same methodology to calculate the pull-up resistor at the input of the Driver (Sink), the minimum
pull-up resistor is:
(5)
The data sheet V
PASS
specification ensures the maximum output voltage is clamped at 3.6 V to support a 3.3-V
connection. Resistors pulling up to 3.3 V on SCL_SINK and SDA_SINK ensure the high level does not exceed
the 3.3-V termination voltage.
Copyright © 2007–2011, Texas Instruments Incorporated 17