UCC5343 Datasheet, PDF (5/6 Pages) Texas Instruments – IrDA Transceiver with Encoder/Decoder

UCC5343
APPLICATION INFORMATION (cont.)
Ground Plane
There are 3 ground connections shown on the applica-
tion drawing. They represent the sensitive analog ground,
the dirty digital ground, and the high current transmitter
ground. These 3 points can simply be geographic group-
ings of connections to a ground plane. If a ground plane
is not used, other provision to isolate the analog and digi-
tal ground currents should be provided. The use of a
ground plane is strongly recommended.
DET Considerations
The DET circuit board trace should be surrounded with
AGND in order to shield leakage noise from DET. The
DET circuit board trace length should be minimized.
Since the PIN diode connected to DET is capacitive,
noise coupling to the cathode of the diode will be cou-
pled directly to DET. For this reason, the 100nF capacitor
on CAT should be located physically close to the cathode
of the PIN diode.
There is natural parasitic coupling from RXX to DET.
RXX should be routed to minimize the parasitic capaci-
tive coupling from RXX to DET.
Analog Power Supply De-coupling
The UCC5343 has a highly sensitive amplifier section ca-
pable of detecting extremely low current levels (200nA
typical). Achieving this sensitivity requires quiet analog
power supply rails. A 100nF high frequency capacitor in
close proximity to AVDD and AGND is required for quiet
analog rails.
The transmitter section of the chip runs from the AVDD
supply and draws high peak currents (~ 500 mA in a typi-
cal application). A bulk capacitor may be required physi-
cally close to the AVDD and AGND pins if the connection
length to the power supply is long or if the supply is or
appears to be relatively high impedance. This bulk ca-
pacitor is in addition to the 100nF high frequency capaci-
tor mentioned above. The bypass capacitors on CAT and
AVDD should present very low equivalent series resis-
tance and inductance to the circuit.
Digital Power Supply
DVDD is fed directly from AVDD through an internal 2k
resistor. The DVDD bypass cap handles all transient cur-
rent produced by the digital section of the chip. If more
drive is required from RXX than the internal 2k resistor
will allow, an external resistor can shunt it. This technique
should always be accompanied by increasing the value
of the de-coupling capacitor on DVDD and AVDD.
Figure 4. IrDA Transceiver with UART Encoder/Decoder Logic
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