Datasheet

TFDU4101
www.vishay.com
Vishay Semiconductors
Rev. 1.7, 29-Jun-2018
6
Document Number: 81288
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RECOMMENDED CIRCUIT DIAGRAM
Operated with a clean low impedance power supply the
TFDU4101 needs no additional external components.
However, depending on the entire system design and board
layout, additional components may be required (see figure
1). That is especially the case when separate power supplies
are used for bench tests. When using compact wiring and
regulated supplies as e. g. in phone applications in most
cases no external components are necessary.
Fig. 1 - Recommended Test Circuit
Note
(1)
R1 is optional when reduced intensity is used
The capacitor C1 is buffering the supply voltage and
eliminates the inductance of the power supply line. This one
should be a Tantalum or other fast capacitor to guarantee
the fast rise time of the IRED current. The resistor R1 is the
current limiting resistor, which may be used to reduce the
operating current to levels below the specified controlled
values for saving battery power.
Vishay's transceivers integrate a sensitive receiver and a
built-in power driver. The combination of both needs a
careful circuit board layout. The use of thin, long, resistive
and inductive wiring should be avoided. The shutdown input
must be grounded for normal operation, also when the
shutdown function is not used.
The inputs (TXD, SD) and the output RXD should be directly
connected (DC-coupled) to the I/O circuit. The capacitor C2
combined with the resistor R2 is the low pass filter for
smoothing the supply voltage. R2, C1 and C2 are optional
and dependent on the quality of the supply voltages V
CC1
and injected noise. An unstable power supply with dropping
voltage during transmission may reduce the sensitivity (and
transmission range) of the transceiver.
The placement of these parts is critical. It is strongly
recommended to position C2 as close as possible to the
transceiver power supply pins.
When extended wiring is used (bench tests!) the inductance
of the power supply can cause dynamically a voltage drop
at V
CC2
. Often some power supplies are not able to follow
the fast current rise time. In that case another 4.7 μF (type,
see table under C1) at V
CC2
will be helpful.
Under extreme EMI conditions as placing an
RF-transmitter antenna on top of the transceiver, we
recommend to protect all inputs by a low-pass filter, as a
minimum a 12 pF capacitor, especially at the RXD port. The
transceiver itself withstands EMI at GSM frequencies above
500 V/m. When interference is observed, the wiring to the
inputs picks it up. It is verified by DPI measurements that as
long as the interfering RF - voltage is below the logic
threshold levels of the inputs and equivalent levels at the
outputs no interferences are expected.
One should keep in mind that basic RF-design rules for
circuit design should be taken into account. Especially
longer signal lines should not be used without termination.
See e.g. “The Art of Electronics” Paul Horowitz, Winfield Hill,
1989, Cambridge University Press, ISBN: 0521370957.
Figure 2 shows an example of a typical application with a
separate supply voltage V
S
and using the transceiver with
the IRED anode connected to the unregulated battery V
batt
.
This method reduces the peak load of the regulated power
supply and saves therefore costs. Alternatively all supplies
can also be tied to only one voltage source. R1 and C1 are
not used in this case and are depending on the circuit
design in most cases not necessary.
In Fig. 2 an option is shown to operate the transmitter at two
different power levels to switch for long range to low power
mode for e.g. saving power for IrDA application but use the
full range specification for remote control. The additional
components are marked in the figure.
For operating at RS232 ports TOIM4232 is recommended
as ENDEC.
V
CC2
, IRED A
V
CC1
Ground
SD
TXD
RXD
IRED C
V
IRED
V
CC
GND
SD
TXD
RXD
R1
(1)
R2
C1
C2
20037
TABLE 1 - RECOMMENDED TESTS AND APPLICATION CIRCUIT COMPONENTS
COMPONENT RECOMMENDED VALUE VISHAY PART NUMBER
C1 4.7 μF, 16 V 293D 475X9 016B
C2 0.1 μF, ceramic VJ 1206 Y 104 J XXMT
R1 Depends on current to be adjusted, e. g. with V
CC2
= 3.3 V 56 Ω is an option for minimum low power operation
R2 47 Ω, 0.125 W CRCW-1206-47R0-F-RT1