October 9, 1997

Low-noise step-up supply
generates ­5.5V output

Larry Suppan, Maxim Integrated Products, Sunnyvale, CA

Some two- to three-cell applications require negative potentials higher than the battery voltage. The typical GaAsFET transmitter in a cellular phone, for example, requires a negative bias of ­5.5V at 5 mA. In the GaAsFET supply of Figure 1, two ICs derive ­5.5V from a battery voltage of 3 to 5.3V. The simple charge-pump voltage doubler, IC₁, applies 6 to 10V to the regulated voltage inverter, IC₂, which also includes a charge pump for voltage conversion. The ICs maintain low switching noise by synchronizing their switching to eliminate beat frequencies: the square-wave output at pin 4 of IC₁, typically 100 kHz, drives the OSC input (pin 4) of IC₂. With the 10-µF capacitor shown, the typical output noise at 5 mA is 2 mV p-p. For input voltages higher than 4V, the circuit can source 20 mA with typical noise of 7 mV p-p.

For V_IN=3.09V and V_OUT=­5.5V, the input to IC₂ is 6.02V, leaving 0.42V of head room in IC₂. That number lets you estimate V_OUT vs V_IN. For V_IN=2.5V, for example, the doubler output is about 4.9V. Subtracting the head room leaves an output voltage of ­4.5V at 5 mA. (DI #2092)

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