900MHz ISM-Band, 250mW Power Amplifiers

with Analog or Digital Gain Control

6

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Detailed Description

The MAX2232/MAX2233 are nonlinear power amplifiers

(PAs) intended for constant envelope applications (FM,

FSK, GMSK). The devices operate over the 902MHz to

928MHz frequency range and provide typically 250mW

of output power and 44% efficiency from a single +2.7V

to +5.5V supply. Both devices typically provide 24dB of

gain, which is adjustable over a continuous 24dB range

through an analog voltage on CTRL of the MAX2232 or

in two 10dB steps through the 2-bit programmable gain

inputs (D0, D1) on the MAX2233. A low-power shutdown

mode on both devices reduces supply current to 0.2µA.

All bias, gain control, and temperature-sensing circuitry

is provided on-chip to save board space and reduce

component count. The signal path includes a variable

gain amplifier (VGA), an output driver, and an output

power transistor. On-chip interstage matching networks

are provided for optimal loading and power transfer.

Internal bias circuitry responds to CTRL and SHDN of

the MAX2232, or to the 2-bit gain/shutdown control

DAC of the MAX2233, providing optimum biasing and

efficiency as well as shutdown and ramp control func-

tions. A unique temperature-sensing circuit protects the

device from overheating by shutting down the PA until

a safe temperature is resumed.

The devices come in a 16-pin PQSOP package, which

incorporates a slug on the bottom of the device to

increase heat conduction and provide a low induc-

tance path to ground.

RF Input Stage (Variable Gain Amplifier)

The input stage includes a variable gain amplifier

(VGA) with 24dB gain-control range. With the appropri-

ate matching network, the input VSWR is nominally

1.5:1 during operation and the change in VSWR is only

1.2:1, relative to normal operation. This stage remains

active during standby mode (MAX2232) to provide a

constant input impedance but is off during shutdown

mode (MAX2232/MAX2233). AC-couple this port with a

DC-blocking capacitor.

RF Second Stage (Driver)

The driver provides gain to overcome interstage match-

ing losses and to produce a signal large enough to

drive the output power transistor into saturation. The

driver stage is off during standby (MAX2232) and shut-

down (MAX2232/MAX2233) modes.

RF Output Stage

(Output Power Transistor)

The output power transistor delivers +24dBm of output

power from a single +3.6V supply. The transistor’s

open-collector output requires a pull-up inductor to

matching network to ensure optimum output power.

Connect the output matching network to RFOUT (pin

8). See the

Typical Application Circuit for bias and

matching components suitable for 900MHz operation.

The ground of the output stage is connected to a metal

slug on the bottom of the MAX2232/MAX2233’s 16-pin

PQSOP package. The metal slug increases heat con-

duction and provides a low-inductance path to ground.

Solder the metal slug to the PC-board ground plane.

___________________________MAX2232

Analog Gain Control

and Power Management

Shutdown Mode

Drive the shutdown (SHDN) pin below 0.6V to place the

MAX2232 in shutdown mode. The VGA, driver, and out-

put transistor are off in shutdown mode and the supply

current is typically reduced to 0.2µA. When SHDN is

above 2.0V, the device is enabled.

Standby Mode

Drive SHDN above 2.0V and CTRL below 0.4V to place

the MAX2232 in standby mode. In standby mode, the

VGA remains biased while the driver and output tran-

sistor are disabled. Supply current is typically 20mA in

standby mode. This mode allows the device to maintain

a good input VSWR while maintaining 38dB of isolation.

This is useful to users who drive the PA with a direct

modulated VCO, where pulling of the VCO results from

changes in the input VSWR.

Analog Gain Control

The voltage applied to the gain-control (CTRL) pin varies

the output power of the MAX2232 continuously over a

24dB range. An internal comparator holds the PA to a

to be set with a resistor-divider or a voltage output DAC.

Output Power Autoramping

The internal bias circuitry of the MAX2232 automatically

ramps the output power of the PA down or up at a con-

stant rate (in W/sec) when switched in or out of shut-

down mode. The output power is ramped to the level set

between RAMP+ and RAMP-. This provides an easy

means of avoiding spectral splatter and complying with

transmit mask requirements, without using a separate

controller. The output power rise and fall time is set by