With battery-powered portable equipment, the right voltage-regulator topology can significantly increase battery operating time on a single charge. Compared to two common topologies—the linear regulator and the step-down regulator—the single-ended primary inductance converter (SEPIC) increases battery operating time on a single charge by up to 50%. (See "Voltage-Regulator Topology In A Nutshell," p. 90.)
Battery voltage is not constant. It can range both above and below the desired output voltage of the regulator. Since the SEPIC configuration provides a regulated output over the entire range of useful battery voltage, it dramatically improves battery operating time.
The following example describes how to design a 3.8-V SEPIC supply for a Global System for Mobile Communications (GSM) power amplifier. Designers can use these guidelines to reduce component size, improve efficiency, and optimize layout. Although this application example is specific to GSM power amplifiers, the concepts also apply to other SEPIC designs.
The GSM power-supply specification requires the same functionality whether the power source is a single lithium-ion (Li-ion) battery or a three-cell nickel-metal-hydride (NiMH) battery (with the cells connected in series). The voltage from a Li-ion battery ranges from 2.7 to 4.2 V, while that of three NiMH batteries ranges from 2.6 to 4.2 V. Therefore, the 3.8-V GSM supply must handle input voltages ranging from 2.6 to 4.2 V.
The average current required from the 3.8-V output is 380 mA, and the power supply must be able to sustain 2.6-A current pulses on an intermittent basis. During shutdown, the output must be dc-isolated from the input so the load doesn't drain the battery. Due to the limited space within a GSM phone, the maximum component height is 4 mm.
This SEPIC power supply surpasses these design objectives (Fig. 1). The input voltage can vary from 2.5 to 5.5 V, and the output current can be as high as 500 mA.
The chief challenge when choosing components that operate over the full range of desired input voltage lies at the low end of the range. The circuit's switching regulator (U2) must start up when the battery is nearly fully discharged. Choosing the right regulator and power MOSFET makes it possible.
Bootstrapping U2—the MAX-669 switching-regulator chip—by connecting its output to its input increases the voltage applied to the gate of power MOSFET Q1, reducing its on-resistance. This reduction aids the startup process and increases efficiency. With bootstrapping, the power supply can start up with lower voltages and heavier loads.
To understand why bootstrapping works, first imagine regulator U2 in a standard step-up configuration. If capacitor C4 were shorted and inductor L2 and diode-pair U1 were removed, the circuit would represent a step-up switching regulator. Next, imagine connecting D1's cathode to U2's VCC pin. By doing so, you've bootstrapped the output to the input.
In this bootstrapped configuration, the battery voltage (minus the voltage drop across D1) powers the chip when it's first starting up. The battery voltage starts to increase as U2 begins boosting. The boosted voltage feeds U2's input, which in turn feeds the gate of MOSFET Q1. The increased voltage enhances the MOSFET, further lowering its on-resistance.
Unfortunately, a SEPIC configuration includes a capacitor (C4) that blocks the path over which the battery finds its way to U2's input (through inductor L1 and catch diode D1). Dual-diode U1 provides an alternate route. Like a standard bootstrapping circuit, U1 allows the battery to power U2 at startup. When U2's output voltage exceeds the battery voltage, the chip, in effect, powers itself, thereby further enhancing the power MOSFET.
Given the 1.5-V typical gate threshold of the Si2302 MOSFET, even the 2.2-V startup voltage (the minimum input voltage minus the 0.3-V diode drop) enhances the n-channel Q1. When the output of the power supply reaches 3.8 V, Q1's gate sees 3.5 V. At 3.5 V, the MOSFET's on-resistance is reduced further, typically to 112 mΩ at 125°C.
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