U2741B-MFB Atmel, U2741B-MFB Datasheet - Page 3

• Manufacturer Part Number: U2741B-MFB
• Manufacturer: Atmel

Specifications of U2741B-MFB

• Operating Frequency (max): 450000kHz
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Operating Supply Voltage (max): 5.5V
• Lead Free Status / Rohs Status: Not Compliant

General Description
The fully integrated VCO and the “single-ended open-
collector” output allow particularly simple, low-cost RF
miniature transmitters to be assembled. The single-ended
output enables a considerably simplified adaptation of
both a magnetic loop antenna of any form or a l/4
antenna. This is because the load impedance must not be
balanced as would be the case with a differential output.
The XTO’s frequency can be selected at either 13.56 MHz
(USA 9.844 MHz) or 6.78 MHz (USA 4.9219 MHz).
Thus, it is possible to use not only exceptionally small SMD
crystals at 13.56 MHz but also very low-cost 6.78-MHz
crystals in a wired metal package (e.g. in the HC49S
housing). The frequency is selected by connecting PIN16
(DIVC) to either GND or V
At high frequencies, crystals have a very fast start-up time
(< 1.5 ms at 13.56 MHz, < 3 ms at 6.78 MHz), whereby
a wait time of 5 to 10 ms is required until the transmitter
IC is locked. This means that the processor does not need
to poll a lock detect output.
Functional Description
The IC can be switched on at both the FSK and the ASK
input. The IC’s ChipSelect is performed by the logical OR
operation of ASK and FSK input. In the case of
V
IS
amplifier and the PLL. The FSK input only activates the
PLL and, if capacitor C3 is installed, pulls the crystal to the
lower frequency, whereby the transmitter is FSK
modulated. After switching on at FSK, the VCO locks onto
the 32 or 64 times higher frequency of the crystal oscillator.
FSK Transmission
The U2741B is switched on by V
is applied to V
means of Pin FSK. This is done by connecting capacitor
C3 in parallel to the load capacitor C4.
ASK Transmission
The U2741B is activated by V
0 V for 5 ms, then the output power can be modulated by
means of Pin ASK. In this case, V
the message, the capacitor C3 is not mounted.
Take-Over of the Clock Pulse in the C
The clock of the crystal oscillator can be used for clocking
the C. The M44C090 and M44C890 have the special
feature of starting with an integrated RC oscillator to
switch on the U2741B with V
3.39-MHz clock frequency is present, so that the message
can be sent with crystal accuracy.
Rev. A2, 22-Dec-00
FSK
off
< 0.35 A. The ASK input activates the power
, V
ASK
ASK
0.3 V, the power-down supply current is
. The output can then be modulated by
S
.
FSK
FSK
FSK
FSK
= V
= V
remains = V
= V
S
S
. 5 ms later, the
S
. V
. 5 ms later, V
ASK
S
remains
during
S
Application Circuit
The following component values are recommendations
for a typical application. C5, C6, C7 are block capacitors.
The values of these capacitors depend on the board
layout. C5 = 1 nF, C6 = 1 nF, C7 = 22 nF are typically used
here. For C5, the impedance between f = 100 MHz and
f = 1 GHz should be as low as possible.
C3 is not needed in ASK transmitter applications. In the
case of FSK, C3 is selected in such a way that the desired
transmission frequency deviation is achieved (typ.
crystal’s load capacity (C4) recommended by the
manufacturer of the crystal. C2 = 3.9 nF, C1 = 15 nF,
R4 = 220 W.
CLoop1, CLoop2 are selected so that the antenna
oscillates in resonance and the adaptation to the
appropriate impedance transformation is possible.
LFeed is an inductor for the antenna’s DC current supply.
A typical value is LFeed = 220 nH. LFeed can be either
printed on the PC_Board or be a discrete component.
Further information regarding the application is provided
in the description of the “RKE Design Kit (U2741B,
U3741B)” in chapter 2.2.2. This chapter labeled
“Application Hints U2741B” also describes the antenna
design in more detail.
Output Power Measurement
The following output network (see figure 4) can be used
for output power evaluation, the exact values of L10, C10
are dependent on the layout.
L10, C10 is the transformation network to adopt the
output impedance of the IC to 50 W. The following table
shows the values for an output power of 2 mW and an
R
PWRSET
30 kHz). The capacitance here depends upon the
f/ MHz
433.92
315
PWRVCC
= 1.2 kW.
Figure 4. Measurement output network
ANT
C10/ pF
2.7
1.8
V
L10/ nH
50 W
L10
C10
S
56
33
U2741B
Z
Load-opt
Z
260 + j330
185 + j268
Load_opt
15009
3 (7)
/ W