TMDS341 Datasheet, PDF (21/25 Pages) Texas Instruments – 3-TO-1 DVI/HDMI SWITCH

www.ti.com
APPLICATION INFORMATION (continued)
Source
V_DDsource
R_upsource
Switch Box
SDAn
TMDS341
TMDS341
SLLS660A – AUGUST 2005 – REVISED OCTOBER 2005
SDA_Sink
V_DDsink
R_upsink
Sink
C_source
C_cable1
C_I
C_O
C_cable2
C_sink
Figure 37. DDC Link from Source to Sink With External Switch Box
R_upsource = 1.5-kΩ pull-up to 5 V
R_upsink = 47-kΩ pull-up to 5 V
R_total = R_upsource // R_upsink = 1.45 kΩ
C_total = C_source // C_cable1 // C_i // C_o // C_cable2 // C_sink
For standard mode I²C, the frequency is at 100 kHz, and the transition time must be less than 1 µs. The total
allowable capacitance, C_total, is then 814-pF. C_source and C_sink are limited by the HDMI specification to 50 pF. C_i/o
for the TMDS341 is 10 pF max. The total capacitance from DVI or HDMI cables, C_cable1 and C_cable2, should then
be less than 704 pF.
Typical capacitance is 200 pF for a 28-AWG 5-m HDMI cable and 300 pF for a 28-AWG 5-m DVI cable. The
recommended total cable length is the length of cable 1, Lcable1, plus the length of cable 2, Lcable2. For a
28-AWG DVI cable, the total cable length is 11 m; and for a 28-AWG HDMI cable, the total cable length is 17
m.
This calculation is applicable to V_IH ≤ V_pass.
Maximum Bus Lengths for DTV Applications
Figure 38 shows the TMDS341 being used as a switch in a DTV and being placed on the same PCB board as
the DVI/HDMI receiver. Unlike Figure 37, the output connector of the TMDS341 stand-alone switch and the input
connector of the sink are removed, which results in a lower capacitance in the DDC link and eliminates the
impedance discontinuity. However, the capacitance of the removed connectors is relatively small, relative to the
total allowable capacitance. The results from the previous section Maximum Bus Lengths for Switch Applications
can be reused if the pull-up resistors and capacitances have the same values. The recommended total cable
length is the length from source to sink.
Source
V_DDsource
Sink
V_DDsink
R_upsource
R_upsink
SDAn
TMDS341
SDA_Sink
DVI/HDMI RX
C_source
C_cable
C_sink
Figure 38. DDC Link From Source to Sink Without External Switch Box
21