300MHz-to-450MHz Low-Power, Crystal-Based

ASK Transmitter

6

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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 floating 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:

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

The PA of the MAX1472 is a high-efficiency, open-drain,

class-C amplifier. With proper output matching network,

the PA can drive a wide range of impedances, including

the small-loop PC board trace antenna and any 50

Ω

antenna. The output-matching network for a 50

Ω anten-

na is shown in the Typical Application Circuit. The out-

put-matching network suppresses the carrier harmonics

and transforms the antenna impedance to an optimal

impedance at PAOUT (pin 4), which is about 250

Ω .

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

PIN

NAME

FUNCTION

1

XTAL1

2

GND

Ground. Connect to system ground.

3

PAGND

Ground for the Power Amplifier (PA). Connect to system ground.

4

PAOUT

Power-Amplifier Output. This output requires a pullup inductor to the supply voltage, which may be part

of the output-matching network to a 50

Ω antenna.

5

ENABLE

Standby/Power-Up Input. A logic low on ENABLE places the device in standby mode.

6

DATA

OOK Data Input. Power amplifier is ON when DATA is high.

Supply Voltage. Bypass to GND with capacitor as close to the pin as possible.

8

XTAL2