“Smartphones, tablet computers, digital cameras, navigation systems, medical devices, and other low-power portable devices often contain multiple integrated circuits fabricated using different semiconductor processes. These devices often require multiple independent power supply voltages, each of which is typically different from that provided by a battery or external AC/DC power supply. Figure 1 shows a typical low-power system powered by a Li-Ion battery.
Smartphones, tablet computers, digital cameras, navigation systems, medical devices, and other low-power portable devices often contain multiple integrated circuits fabricated using different semiconductor processes. These devices often require multiple independent power supply voltages, each of which is typically different from that provided by a battery or external AC/DC power supply.
Figure 1 shows a typical low-power system powered by a Li-Ion battery. The battery’s usable output range is 3 V to 4.2 V, while the IC requires 0.8 V, 1.8 V, 2.5 V, and 2.8 V. A simple way to reduce the battery voltage to a lower DC voltage is to use a low dropout regulator (LDO). However, when VIN is much higher than VOUT, the power not delivered to the load is lost as heat, resulting in inefficient LDOs. A common alternative is to use a switching converter, which alternately stores energy in the magnetic field of an Inductor and then releases it to the load at different voltages. This scheme has lower losses and is a better choice for high-efficiency operation. This article describes a step-down converter, which provides a lower output voltage. Boost converters, which are described in a separate article, provide higher output voltages. Switching converters with built-in FETs as switches are called switching regulators, and switching converters that require external FETs are called switching controllers. Most low-power systems use both LDOs and switching converters to achieve cost and performance goals.
Figure 1. Typical Low Power Portable System
A buck regulator consists of 2 switches, 2 capacitors, and 1 inductor, as shown in Figure 2. The non-overlapping switch drive mechanism ensures that only one switch is on at any one time, avoiding undesirable current “shoot-through”. In phase 1, switch B is open and switch A is closed. The inductor is connected to VIN, so current flows from VIN to the load. Since there is a positive voltage across the inductor, the current increases. In phase 2, switch A is open and switch B is closed. The inductor is connected to ground, so current flows from ground to the load. Since there is a negative voltage across the inductor, the current decreases and the energy stored in the inductor is released into the load.
Figure 2. Buck Converter Topology and Operating Waveforms
Note that switching regulators can operate either continuously or intermittently. Continuous conduction When operating in continuous conduction mode (CCM), the inductor current does not drop to zero; when operating in discontinuous conduction mode (DCM), the inductor current can drop to zero. Low-power buck converters rarely operate in discontinuous conduction mode. As designed, the current ripple (shown as ΔI in Figure 2) is typically 20% to 50% of the nominal load current.
In Figure 3, Switch A and Switch B are implemented with PFET and NFET switches, respectively, to form a synchronous buck regulator. The term “synchronous” refers to the use of a FET as a low-side switch. Buck regulators that use Schottky diodes instead of low-side switches are called “asynchronous” (or non-synchronous) types. Synchronous buck regulators are more efficient when handling low power because the FET’s voltage drop is lower than that of a Schottky diode. However, if the bottom FET is not released when the inductor current reaches zero, the light-load efficiency of the synchronous converter will be reduced, and the extra control circuitry will increase the complexity and cost of the IC.
Figure 3. Buck Regulator Integrated Oscillator, PWM Control Loop, and Switching FET
Current low-power synchronous buck regulators use pulse-width modulation (PWM) as the main operating mode. The PWM keeps the frequency constant and adjusts the output voltage by changing the pulse width (tON). The average power delivered is proportional to the duty cycle D, so this is an efficient way to add power to the load.
The FET switches are controlled by a pulse-width controller that uses voltage or current feedback in the control loop to regulate the output voltage in response to load changes. Low-power buck converters typically operate in the frequency range of 1 MHz to 6 MHz. At higher switching frequencies, smaller inductors can be used, but doubling the switching frequency reduces efficiency by about 2%.
At light loads, PWM operation does not always improve system efficiency. Taking the graphics card power supply circuit as an example, when the video content changes, the load current of the buck converter driving the graphics processor also changes. Continuous PWM operation can handle a wide range of load currents, but at light loads, the power required by the regulator can take up a large percentage of the total power delivered to the load, resulting in a rapid loss of system efficiency. For portable applications, buck regulators integrate other power saving techniques such as pulse frequency modulation (PFM), pulse skipping, or a combination of the two.
Analog Devices defines a high-efficiency light-load operating mode as a “Power Save Mode” (PSM). When entering power saving mode, the PWM regulation level will be offset, causing the output voltage to rise until it reaches a level about 1.5% higher than the PWM regulation level, at which point the PWM operating mode is turned off and both power switches are disconnected , the device enters idle mode. COUT can discharge until VOUT drops to the PWM regulation voltage. The device then drives the inductor, causing VOUT to rise again to the upper threshold. This process repeats as long as the load current is below the power save mode current threshold.
The ADP2138 is a compact 800 mA, 3 MHz, step-down DC-DC converter. Figure 4 shows a typical application circuit. Figure 5 shows the efficiency improvement in forced PWM operation and automatic PWM/PSM operation. PSM interference can be difficult to filter due to frequency variations, so many buck regulators provide a MODE pin (shown in Figure 4) that the user can use to force the device to operate in continuous PWM mode, or allow the device to operate in automatic PWM/PSM mode operation. The MODE pin can either be hardwired to set any operating mode, or it can be dynamically switched as needed to save power.
Figure 4. ADP2138/ADP2139 Typical Application Circuit
Figure 5. Efficiency of ADP2138: (a) Continuous PWM Mode; (b) PSM Mode
Buck Regulators Improve Efficiency
Battery life is a feature of high concern in the design of new portable devices. Improving system efficiency can extend battery life and reduce the frequency of replacement or charging. For example, a rechargeable Li-Ion battery can drive a 500 mA load at 0.8 V using the ADP125 LDO, as shown in Figure 6. The efficiency of this LDO is only 19% (VOUT/VIN × 100% = 0.8/4.2 × 100%). The LDO cannot store unused energy, so the remaining 81% of the power (1.7 W) can only be dissipated inside the LDO as heat, which can cause a rapid temperature rise in the handheld device. If the ADP2138 switching regulator is used, the operating efficiency will be 82% at 4.2 V input and 0.8 V output, which is more than 4 times higher than the efficiency of the previous solution, and the temperature rise of the portable device will be greatly reduced. These dramatic improvements in system efficiency have led to the widespread adoption of switching regulators in portable devices.
Buck Converter Key Specifications and Definitions
Input voltage range: The input voltage range of a buck converter determines the lowest usable input supply voltage. Specifications may provide a wide input voltage range, but VIN must be higher than VOUT for efficient operation. For example, to get a stable 3.3 V output voltage, the input voltage must be higher than 3.8 V.
Ground Current or Quiescent Current: IQ is the DC bias current not delivered to the load. The lower the IQ of the device, the higher the efficiency. However, IQ can be specified for many conditions, including shutdown, zero load, PFM operation, or PWM operation. Therefore, in order to determine the best buck regulator for an application, it is best to look at actual operating efficiency data for a specific operating voltage and load current.
Shutdown Current: This is the input current consumed by the device when the enable pin is disabled, typically well below 1µA for low power buck regulators. This metric is important for the ability of the battery to have a long standby time when the portable device is in sleep mode.
Output Voltage Accuracy: ADI’s buck converters have high output voltage accuracy, with fixed output devices trimmed to within ±2% (25°C) at the factory. Output voltage accuracy is specified over operating temperature, input voltage, and load current range, with worst-case inaccuracy specified as ±x%.
Line regulation: Line regulation refers to the rate of change of the output voltage with the change of the input voltage under the rated load.
Load Regulation: Load regulation refers to the rate of change of the output voltage as the output current changes. Most buck regulators keep the output voltage essentially constant for slowly varying load currents.
Load Transients: Transient errors can occur if the load current changes rapidly from a lower level to a higher level, causing the operating mode to switch between PFM and PWM, or from PWM to PFM. Not all data sheets specify load transients, but most data sheets provide load transient response curves for various operating conditions.
Current Limiting: Buck regulators such as the ADP2138 have built-in protection circuits that limit the forward current flowing through the PFET switch and synchronous rectifier. Positive current control limits the amount of current that can flow from the input to the output. The negative current limit prevents the inductor current from reversing and flowing out of the load.
Soft-Start: The internal soft-start function is very important for the buck regulator, it controls the output voltage ramp-up at start-up, thus limiting the inrush current. This prevents the input voltage from drooping when a battery or high impedance source is connected to the converter input. After the device is enabled, the internal circuitry begins the power-up cycle.
Start-up time is the time from the rising edge of the enable signal to when VOUT reaches 90% of its nominal value. This test is usually performed with VIN applied and the enable pin toggled from off to on. With the enable pin tied to VIN, the start-up time may increase significantly when VIN is switched from off to on because the control loop needs some settling time. In portable systems where the regulator needs to be turned on and off frequently to save power, the start-up time of the regulator is an important consideration.
Thermal Shutdown (TSD): Thermal shutdown circuitry shuts down the regulator when the junction temperature exceeds specified limits. Extreme junction temperatures can be caused by high operating current, poor board cooling, or high ambient temperature. The protection circuit includes some hysteresis to prevent the device from returning to normal operation until the die temperature falls below a preset limit.
100% Duty Cycle Operation: As VIN falls or ILOAD rises, the buck regulator reaches a limit: even though the PFET switch is turned on at 100% duty cycle, VOUT is still lower than the expected output voltage. At this point, the ADP2138 smoothly transitions to a mode that keeps the PFET switch on at 100% duty cycle. When the input condition changes, the device restarts PWM regulation immediately, and VOUT does not overshoot.
Discharge Switch: In some systems, if the load is very light, the output of the buck regulator may remain high for some time after the system enters sleep mode. However, if the system initiates a power-up sequence before the output voltage discharges, the system may latch up or cause damage to the device. The ADP2139 buck regulator discharges the output through an integrated switch resistor (100 Ω typical) when the enable pin goes low or the device enters undervoltage lockout/thermal shutdown.
Under-Voltage Lockout: Under-Voltage Lockout (UVLO) ensures that voltage is output to the load only when the system input voltage is above a specified threshold. UVLO is important because it only powers up the device when the input voltage reaches or exceeds the voltage required for stable operation of the device.
Low-power buck regulators demystify switching DC-DC converter design. Analog Devices offers a line of highly integrated, rugged, easy-to-use, cost-effective buck regulators that require very few external components to achieve high efficiency.System designers can use the design calculations provided in the application section of the data sheet, or use the ADIsimPower™ design tool