The function of a DC-DC switching converter is to efficiently convert one DC voltage into 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 a buck/boost DC-DC converter and won't go into details here.)

    Figure 1 shows a typical low-power system powered by a single-unit lithium-ion battery. The battery's available output ranges from about 3.0V when discharged to 4.2V when fully charged. The system IC requires voltages of 1.8V, 3.3V, and 3.6V for optimal operation. Lithium-ion batteries start with a voltage of 4.2V and end with a voltage of 3.0V, during which a step-down/boost regulator can provide a constant voltage of 3.3V, while a step-down regulator or low-voltage difference regulator (LDO) can provide a voltage of 1.8V when the battery is discharged. In theory, when the battery voltage is higher than 3.5V, a step-down regulator or LDO can be used to generate 3.3V voltage, but when the battery voltage drops below 3.5V, the system will stop working. Allowing the system to shut down too early reduces the amount of time the system can operate before the battery needs to be recharged.

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Figure 1. Typical low-power portable system

    The buck/boost regulator has four internal switches, two capacitors, and an inductor, as shown in Figure 2. Today's low-power, high-efficiency buck/boost regulators can reduce losses and increase efficiency by actively operating two of the switches when operating in buck or boost mode.

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Figure 2. Topology of the buck/boost converter

    When VIN is greater than VOUT, switch C is turned off and switch D is turned off. Switches A and B work as in standard step-down regulators, as shown in Figure 3.

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Figure 3.Buck mode when VIN > VOUT

    When VIN is less than VOUT, switch B is turned off and switch A is turned off. Switches C and D work the same way as in the boost regulator, as shown in Figure 4. The most difficult mode of operation is when the VIN is within the VOUT ± 10% range, at which point the regulator goes into buck/boost mode. In buck/boost mode, two operations (buck and boost) occur in a single switching cycle. Special attention should be paid to reducing losses, optimizing efficiency, and eliminating instability due to mode switching. The goal is to keep the voltage stable and the current ripple in the inductor to a minimum, ensuring good transient performance.

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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 maximum battery life for portable applications, a power-saving mode is also used to reduce switching frequency at light loads. For wireless and other low-noise applications, variable frequency power-saving modes can cause interference, and by adding logic control inputs, the converter can be forced to operate in fixed-frequency PWM mode under all load conditions.

Buck/boost regulators increase system efficiency

    Many portable systems today are powered by single-cell lithium-ion rechargeable batteries. As mentioned above, the battery starts at 4.2V on a full charge and slowly discharges to 3.0V. When the battery output drops below 3.0V, the system shuts down, preventing the battery from being damaged by excessive discharge. When the 3.3V voltage rail is generated by the low voltage difference regulator, the system will be in

VIN MIN = VOUT + VDROUPOUT = 3.3 V + 0.2 V = 3.5 V 

    At this time, only 70% of the energy stored in the battery is used. However, if a buck/boost regulator is used, such as the ADP2503 or ADP2504, the system can operate continuously up to the minimum actual battery voltage. The ADP2503 and ADP2504 (see Appendix) are high efficiency, 600 mA and 1000 mA low static current, buck/boost DC-DC converters that operate at an input voltage above, below, or equal to the regulated output voltage. The power switch is built in to minimize the number of external components and the area of the printed circuit board (PCB). In this way, the system can operate up to 3.0V, thus making full use of the energy stored in the battery and increasing the system's operating time before the battery needs to be recharged.

    To conserve power in portable systems, various subsystems (such as the microprocessor, display backlight, and power amplifier) frequently switch between full on and sleep mode when not in use, resulting in large voltage transients along the battery power line. These transients cause the battery output voltage to drop below 3.0V for a short time and trigger a low battery warning, which shuts down the system before the battery is fully discharged. The buck/boost solution can withstand voltage swings as low as 2.3V, helping to maintain the system's potential operating time.

Buck/boost regulator main specifications characteristics and definitions

    Output voltage range options: Step-down/boost regulators provide a fixed output voltage rating, or an option to allow the output voltage to be programmed via an external resistance divider.

    Ground current or static current: The lower the Iq of a DC bias current (Iq) device that is not delivered to the load, the more efficient it is; however, the Iq can be specified for many conditions, including off, load, pulse frequency (PFM) operating mode, or pulse width (PWM) operating mode. Therefore, in order to determine the best boost regulator for an application, it is best to look at the actual operating efficiency of a specific operating voltage and load current. Settings.

    Turn-off current: This is the input current consumed by the device when the enable pin is disabled. Low Iq is important for the ability of battery-powered devices to stand still for a long time in sleep mode. During a logic-controlled shutdown, the input is disconnected from the output and the current drawn from the input source is less than 1 μA.

    Soft start: It is important to have a soft start function, and the output voltage rises slowly in a controlled manner, so as to avoid the overshoot of the output voltage when starting.

    Switching frequency: Low power buck/boost converters generally operate in the frequency range of 500 kHz to 3 MHz. When the switching frequency is higher, the inductance used can be smaller and the PCB area can be reduced, but each doubling of the switching frequency reduces the efficiency by about 2%.

    Thermal shutoff (TSD): When the junction temperature exceeds the specified limit, the thermal shutoff circuit turns off the regulator. Consistently high junction temperatures can be caused by high operating currents, poor board cooling, and/or high ambient temperatures. The protection circuit includes hysteresis, so after a thermal shutdown occurs, the device will not return to normal operation until the on-chip temperature drops below the preset limit.

Closing remarks

    Low power buck/boost regulators make it easy to design DC-DC switching converters with proven, reliable performance and deep support. Analog Devices not only provides a comprehensive data book and lists the design calculations in its applications section, but also provides the ADIsimPower design tool to simplify the task for the end user.