TFDU4101

www.vishay.com

Vishay Semiconductors

Rev. 1.6, 04-Jul-12

6

Document Number: 81288

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT

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

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 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

the fast current rise time. In that case another 4.7 μF (type,

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

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 figure 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

V

CC1

Ground

SD

TXD

RXD

IRED

C

V

IRED

V

CC

GND

SD

TXD

RXD

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

R2

47

, 0.125 W

CRCW-1206-47R0-F-RT1