MAX1472
300MHz-to-450MHz Low-Power,
Crystal-Based ASK Transmitter
6
Detailed Description
The MAX1472 is a highly integrated OOK/ASK transmit-
ter operating over the 300MHz to 450MHz frequency
range. The IC includes a complete PLL and a highly
efficient PA. The device can also be easily placed into
a 5nA low-power shutdown mode.
Shutdown Mode
The ENABLE pin is internally pulled down with a 15µA
current source. If the pin is left unconnected or pulled
low, the MAX1472 goes into shutdown mode, where the
supply current drops to less than 5nA. When ENABLE
is high, the IC is enabled and is ready for transmission
after 220µs (frequency settles to within 50kHz).
The 220µs turn-on time of the MAX1472 is mostly domi-
nated by the crystal oscillator startup time. Once the
oscillator is running, the 1.6MHz PLL loop bandwidth
allows fast-frequency recovery during power-amplifier
toggling.
Phase-Locked Loop
The PLL block contains a phase detector, charge
pump, integrated loop filter, VCO, 32X clock divider,
and crystal oscillator. This PLL requires no external
components, other than a crystal. The relationship
between the carrier and crystal frequency is given by:
f
XTAL
= f
RF
/ 32
The lock-detect circuit prevents the PA from transmit-
ting until the PLL is locked. In addition, the device shuts
down the PA if the reference frequency is lost.
Power Amplifier (PA)
The PA of the MAX1472 is a high-efficiency, open-drain,
switch-mode amplifier. With proper output matching net-
work, the PA can drive a wide range of impedances,
including the small-loop PC board trace antenna and
any 50Ω antenna. The output-matching network for a
50Ω antenna is shown in the
Typical Application Circuit
.
The output-matching network suppresses the carrier har-
monics and transforms the antenna impedance to an
optimal impedance at PAOUT (pin 4), which is between
100Ω and 150Ω for a 2.7V supply.
When the output matching network is properly tuned,
the PA transmits power with high efficiency. The
Typical
Application Circuit
delivers 10.3dBm at 2.7V supply with
9.1mA of supply current. Thus, the overall efficiency is
44%. The efficiency of the PA itself is more than 52%.
Applications Information
Output Power Adjustment
It is possible to adjust the output power down to
-10dBm with the addition of a resistor. The addition of
the power-adjust resistor also reduces power con-
sumption. See the Supply Current and Output Power
vs. External Resistor and Supply Current vs. Output
Power graphs in the
Typical Operating Characteristics
section. It is imperative to add both a low-frequency
and a high-frequency decoupling capacitor as shown
in the
Typical Application Circuit
.
Pin Description
