Three basic technologies for high-efficiency DC-DC converters

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.

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.

Buck/Boost Regulators Improve System Efficiency

Many of today's portable systems are powered by a single-cell lithium-ion rechargeable battery. As mentioned above, the battery starts at 4.2 V when fully charged and slowly discharges to 3.0 V. When the battery output drops below 3.0 V, the system shuts down to protect the battery from damage due to excessive discharge. When a low-dropout regulator is used to generate the 3.3 V rail, the system shuts down.

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

The ADP2503 and ADP2504 (see Appendix) are high-efficiency, 600-mA and 1000-mA low-quiescent-current, buck/boost dc-to-dc converters that can operate with input voltages above, below, or equal to the regulated output voltage. The power switch is internal, minimizing the number of external components and printed circuit board (PCB) area. With this approach, the system can operate all the way down to 3.0 V, fully utilizing the stored energy in the battery and increasing the system operating time before the battery needs to be recharged.

To save power in portable systems, various subsystems (such as microprocessors, display backlights, and power amplifiers) frequently switch between full-on and sleep modes when not in use, causing large voltage transients on the battery power line. These transients can cause the battery output voltage to drop below 3.0 V for a short period of time and trigger a low-battery warning, causing the system to shut down before the battery is fully discharged. Buck-boost solutions can withstand voltage swings as low as 2.3 V, helping to maintain the system's potential operating time.

Buck/Boost Regulator Key Specifications and Definitions

Output Voltage Range Options: Buck/Boost regulators are available with nominal fixed output voltages or with options that allow the output voltage to be programmed via an external resistor divider.

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

Shutdown Current: This is the input current consumed by the device when the enable pin is disabled. Low Iq is important for battery-powered devices to be able to operate in sleep mode for long periods of time. During the logic-controlled shutdown period, the input is disconnected from the output and less than 1 μA is drawn from the input source.

Soft start: It is important to have a soft start function, which allows the output voltage to rise slowly in a controlled manner to avoid output voltage overshoot during startup.

Switching frequency: Low-power buck/boost converters typically operate in the 500 kHz to 3 MHz range. Higher switching frequencies allow for smaller inductors and reduced PCB area, but efficiency decreases by about 2% for every doubling of the switching frequency.

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