Low-Noise, Regulated, -2V GaAsFET Bias

The MAX840/MAX843/MAX844 low-noise, inverting charge-pump power supplies are ideal for biasing GaAsFETs in cellular telephone transmitter amplifiers. They operate with inputs down to 2.5V.

The MAX840 offers both a -2V preset output and a -0.5V to -9.4V adjustable output. The MAX843/MAX844 use an external positive control voltage to set the negative output voltage. Input voltage range for all the devices is 2.5V to 10V, and output current is 4mA with VIN > 2.7V. These circuits can operate with small capacitors, as low as 0.22µF.

An internal linear regulator reduces the MAX840's output voltage ripple to 1mVp-p. With a well-filtered control voltage (VCTRL), the MAX843/MAX844 also achieve less than 1mVp-p typical output ripple. Supply current is 750µA, and reduces to less than 1µA in shutdown (MAX840/MAX843). The MAX844's unregulated output is active in shutdown, with the charge pump switching at 20kHz. It provides a low-power LCD supply.

Applications

* Cell Phones * Continuously Adjustable GaAsFET Bias * GaAsFET Power Amplifier Modules * LCD-Bias Contrast Control * Personal Communicators, PDAs * Regulated Negative Power Supplies * Wireless Data Loggers

* Fixed -2V or Adjustable -0.5V to -9.4V Output at 4mA (MAX840)

* 2.5V to 10V Input Voltage Range

* Operate with Small Capacitors (as low as 0.22µF)

* 1mVp-p Output Voltage Ripple

* Charge-Pump Switching Frequency:

* 100kHz in Normal Operation

* 20kHz in Shutdown Mode (MAX844)

* 1µA Max Logic-Level Shutdown Over Temp. (MAX840/MAX843)

* Small 8-Pin SO Package