Single Boost Converter Builds Dual Polarity Supply

Sep 1, 2006 12:00 PM
By Yogesh Sharma, Applications Engineer, Standard Power Products Group, Analog Devices, San Jose, Ca

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Many portable devices use LCD displays. These displays need large positive and negative power supplies, but most mobile devices have power rails at 5 V, 3.3 V or even lower. Thus, the challenge is not only to boost the input power supply, typically between 3 V and 5 V, to around +12 V, but also to generate the negative voltage rail, usually -6 V. Typically, the +12-V rail is generated by a boost converter, such as Analog Devices' ADP1611 (Fig. 1).

The ADP1611 consists of a power switch, a PWM controller, a reference voltage source and an error amplifier. When the switch (internal to the ADP1611) turns on, the SW node is grounded, leading to current buildup in the inductor. When the switch turns off, the stored inductor current flows through the diode D_OUT+ to the output capacitor and load, generating an output voltage greater than the input voltage. The error amplifier in the ADP1611 measures the output voltage through a voltage divider and creates an error signal that allows the PWM controller to drive the switch with the correct on and off times to maintain the desired output voltage and load current.

The output voltage is set by the ratio of the resistor divider formed by resistors R1 and R2 according to the following equations:

The value of the input and output capacitor are fixed at 10 µF, while the size of the inductor changes with the frequency at which the PWM controller is switching, which is set by the logic pin RT. Also, the diode D_OUT+ should be chosen such that it can conduct the average output current.

A boost converter can also be used to generate a negative voltage rail by using a charge-pump configuration (Fig. 2). In this circuit, when the switch turns off, the inductor dumps current into charge-pump capacitor C1 and diode D1 to ground. When the switch turns on, the voltage on capacitor C1 forward biases diode D2. The charge held in C1 transfers through diode D2 to the output capacitor (C_OUT-), which draws current out of the load. The on and off times of the switch can be modulated to regulate the negative output voltage rail.

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