Get buck-boost performance from a boost regulator

Tom Gross, Linear Technology

EDN

The SEPIC (single-ended, primary-inductance-converter) topology is generally a good choice for voltage regulators that must produce an on output voltage that falls in the middle of the input-voltage range, such as a 5 V output from a 2.7 to 6 V input. The topology has some disadvantages, however. The efficiency of a SEPIC circuit fares worse than that of buck and boost regulators, and SEPIC designs often involve the use of large, complex magnetic components, thereby complicating the design task. Figure 1 shows a simple and efficient alternative topology. When the input voltage is lower than the output voltage, the circuit operates as a normal boost regulator. The inductor, L₁, stores energy when switch Q₁ is on and the boost diode, D₁, is reverse-biased. While D₁ is off, the output capacitor, C₁, delivers the load current. When Q₁ turns off, L₁ reverses its polarity, thereby forward-biasing D₁. L₁ then charges C₁ and delivers current to the load. The inductor voltage adds to the input voltage to generate the output voltage.

Figure 1.	This circuit can both boost and step down the input voltage, depending on whether the input voltage is lower or higher than the output voltage.

The low-battery comparator in IC₁, which usually checks battery levels, monitors the output voltage through its LBI pin. IC₁'s internal comparator output switches low (sinking current). This action turns the p-channel Q₁ fully on, creating a low-impedance path to the output. When the input voltage is the same value as or greater than the output voltage, the circuit functions like a linear regulator. In this case, the internal comparator's output assumes a high-impedance state. The voltage at the gate of Q₁ begins to pull up through R₁, a 220-kΩ resistor, so Q₁ begins to turn off. This action forces the output voltage to decrease, and the comparator eventually again switches states from high to low (sinking current). The comparator's low state causes the output voltage to rise again, and the cycle repeats. Thus, the circuit begins to operate as a linear regulator, with Q₁ acting as the pass transistor.

Figure 2.	The efficiency of the circuit in Figure 1 depends on whether the circuit acts as a boost converter or a linear regulator.

The circuit can also disconnect the input-to-output current path, unlike a conventional boost regulator. The shutdown signal (/V_SHDN) connects to the gate of Q₂, a logic-level, p-channel MOSFET, as well as to IC₁'s shutdown (/SD) pin. When the shutdown signal goes low, it turns off IC₁ and turns on Q₂. This action delivers V_IN (via R₂, a 100 Ω resistor) to the gate of Q₁, thereby turning off the transistor. Hence, the shutdown operation disconnects the input-to-output current path. Figure 2 shows how the efficiency of this linear-boost regulator depends on its mode of operation. When the input voltage is lower than the output voltage, the efficiency of the regulator is that of a boost regulator. When the input voltage exceeds the output voltage, the circuit operates as a linear regulator in which efficiency is approximately V_OUT/V_IN.

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