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GaAsFET_BIAS_SUPPLY

2015-03-24 03:53  
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GaAsFET_BIAS_SUPPLY
Cellular telephones and other mobile communications gear use depletion-mode GaAsFET RE output transistors, which require a negative bias voltage for proper operation. This LTC1261 circuit generates a regulated negative voltage using no inductors, and will give plenty of drive for GaAsFET bias circuits. The LTC1261CS8 inverts the supply voltage (5 V to -4.5 V), whereas the LTC1261CS can invert or double the supply voltage (3 V to -4 or -5 V). Both parts regulate the generated output voltage. The quiescent current is just 600 μA, and the shutdown current is only 5 μA. The combination of high oscillator frequency, which reduces the switched capacitor size, and the variety of fixed-output regulated negative voltages allows for a minimum-space power supply. The LTC1261 comparator output controls a P-channel MOSFET to ensure that the drain current for the GaAsFET is switched off until the regulated negative output voltage is valid. This ensures that the gate voltage is sufficient to keep the GaAsFET off during power-up, preventing unsaturated operation and excessive operation.