Low-Noise, Dual-Output, Regulated Charge Pump for GaAsFET, LCD, and VCO Supplies

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The MAX768 low-noise, dual-output, regulated charge pump provides a negative output for biasing GaAsFET power amplifiers, and a positive output for powering voltage-controlled oscillators (VCOs) in wireless handsets. The outputs can also be used to power LCDs. Output ripple is less than 2mVp-p. The MAX768 is intended for use in low-voltage systems where a simple charge-pump inverter is inadequate, or where the VCO needs more range to improve its signal-to-noise ratio. The input range is 2.5V to 5.5V, enabling direct power from 1Li+ and 3-cell NiMH/NiCd batteries.

The MAX768 includes a voltage-doubler charge pump, followed by an inverting charge pump. This combination produces unregulated outputs that are ±2x the input. Two internal low-dropout linear regulators provide the low-noise, regulated positive and negative outputs. Output current is guaranteed to be at least 5mA per output. The linear regulators use CMOS devices, so the quiescent current remains independent of output loading (even in dropout), and the dropout voltage approaches zero with no load current.

The MAX768 has two preset switching frequencies (25kHz or 100kHz), or can be synchronized by an external clock from 20kHz to 240kHz. This flexibility permits users to optimize their designs based on noise, capacitor size, and quiescent-supply-current criteria.

The device features Dual Mode™ operation: the output voltage is preset to +5V and -5V, or can be adjusted by adding external resistor dividers. Other features include independent shutdowns and a logic output that signals when the negative voltage has risen to within 10% of its regulation setpoint (to protect the power amplifier GaAsFET). The MAX768 is available in a space-saving, 16-pin QSOP, which is the same size as a standard 8-pin SO.

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