DC-DC Buck/Boost Regulator Design Solution

The job of a DC-DC switching converter is to efficiently convert one DC voltage to another. High-efficiency DC-DC converters use three basic technologies: buck, boost, and buck/boost. Buck converters are used to produce low DC output voltages, boost converters are used to produce high DC output voltages, and buck/boost converters are used to produce output voltages that are less than, greater than, or equal to the input voltage. (This article will focus on how to successfully apply buck/boost DC-DC converters and will not be discussed here.)

Figure 1 shows a typical low-power system powered by a single-cell lithium-ion battery. The usable output of the battery ranges from about 3.0 V when discharged to 4.2 V when fully charged. System ICs require 1.8 V, 3.3 V, and 3.6 V for optimal operation. The lithium-ion battery starts at 4.2 V and ends at 3.0 V, during which a buck/boost regulator can provide a constant 3.3 V, while a buck regulator or low-dropout regulator (LDO) can provide 1.8 V as the battery discharges. In theory, a buck regulator or LDO can be used to generate 3.3 V when the battery voltage is above 3.5 V, but the system will stop operating when the battery voltage drops below 3.5 V. Allowing the system to shut down prematurely reduces the system operating time before the battery needs to be recharged.

Figure 1. Typical low-power portable system.

Buck/boost regulators consist of four switches, two capacitors, and an inductor, as shown in Figure 2. Today’s low-power, high-efficiency buck/boost regulators only actively operate two of the switches when operating in buck or boost mode, which reduces losses and improves efficiency.

Figure 2. Buck-boost converter topology.

When VIN is greater than VOUT, switch C opens and switch D closes. Switches A and B operate the same as in a standard buck regulator, as shown in Figure 3.

Figure 3. Buck mode when VIN > VOUT

When VIN is less than VOUT, switch B is open and switch A is closed. Switches C and D operate in the same manner as in the boost regulator, as shown in Figure 4. The most difficult operating mode is when VIN is within VOUT ± 10%, when the regulator enters buck/boost mode. In buck/boost mode, both operations (buck and boost) occur in one switching cycle. Special attention should be paid to reducing losses, optimizing efficiency, and eliminating instability caused by mode switching. The goal is to maintain voltage regulation, minimize current ripple in the inductor, and ensure good transient performance.

Figure 4. BoostVIN

For high load currents, the buck/boost regulator uses current mode, fixed frequency, pulse width modulation (PWM) control for excellent stability and transient response. To ensure the longest battery life in portable applications, a power save mode is also used to reduce the switching frequency at light loads. For wireless applications and other low-noise applications, the variable frequency power save mode may cause interference, and a logic control input can be added to force the converter to operate in a fixed frequency PWM mode under all load conditions.