MAXIM MAX881REUB+T

The MAX881R low-noise, inverting power supply is designed for biasing GaAsFET power amplifiers in portable wireless applications. This device is a chargepump inverter followed by a negative linear regulator. The input voltage range is 2.5V to 5.5V. The output is preset at -2.0V or can be set, using two resistors, to any voltage from -0.5V to (-VIN + 0.6V). It can deliver up to 4mA. The internal linear regulator also filters the output to 1mVp-p ripple and noise.

Other features include a power-OK (POK) output that signals when the negative voltage is within 7.5% of its set point. It protects the GaAsFET by not allowing power to be applied to the GaAsFET’s drain until it is properly biased. The signal can be routed either to a microcontroller or directly to a switch at the GaAsFET drain. The MAX881R is available in a 10-pin μMAX package.

The MAX881R, a low-noise, inverting, regulated chargepump power supply, is designed for biasing GaAsFET devices such as power-amplifier modules in cellular handsets.

The applied input voltage (VIN) is inverted to a negative voltage at NEGOUT by a capacitive charge pump. This voltage is regulated by an internal linear regulator at OUT. With FB connected to GND, VOUT is regulated at -2V. Alternatively, use a voltage-divider at FB to adjust the output voltage between -0.5V and -(VIN - 0.6V). The internal linear regulator reduces the ripple noise induced by the charge-pump inverter to 1mVp-p at OUT. In addition, the excellent AC rejection of the linear regulator attenuates noise from the incoming supply.

The MAX881R has an active-low, open-drain, power-OK (POK) output. This output goes low when OUT reaches 92.5% of the regulated output voltage. POK can be used to drive the gate of a P-MOSFET that switches power to the GaAsFET power amplifier, thereby ensuring that the power amplifier is not powered before the required negative bias voltage is present.

The MAX881R features a shutdown mode that reduces supply current to less than 1μA.

• Small µMAX Package
• 1mVp-p Output Voltage Ripple and Noise
• Power-OK Signal to Control GaAsFET Drain Switch
• 0.05μA Logic-Controlled Shutdown
• 1ms Guaranteed Startup
• 2.5V to 5.5V Input
• -0.5V to (-VIN + 0.6V) Output at up to 4mA
• Operates with One 4.7μF and Three 0.22μF Capacitors (no inductors needed)

• Cell Phones
• Wireless Modems
• PCS Phones
• Two-Way Pagers
• PHS Phones
• Mobile Radios
• Wireless Handsets
• Wireless Computers