Power for portable electronics such as smartphones, GPS navigation systems, and tablet computers can come from low-voltage solar panels, batteries, or AC-DC power. Battery-powered systems often stack batteries in series to achieve higher voltages, but this technique is not always possible due to lack of space. Switching converters use an inductive magnetic field to alternately store electrical energy and discharge it to the load at different voltages. Because the losses are low, it is a good and efficient choice. A capacitor connected to the output of the converter reduces output voltage ripple. The boost converter discussed in this article provides a higher voltage; while the buck converter discussed in the previous article 1 provides a lower output voltage. Switching converters with built-in FETs as switches are called switching regulators,2 switching converters that require external FETs are called switching controllers.3
Figure 1 shows a typical low-power system powered by two AA batteries connected in series. The electrically usable output range is approximately 1.8 V to 3.4 V, while the IC requires 1.8 V and 5.0 V for operation. Boost converters can boost voltage without increasing the number of battery cells to power WLED backlights, miniature hard drives, audio devices and USB peripherals, while buck converters can power memory and displays.
Figure 1. Typical Low Power Portable System
The inductance’s tendency to resist current changes provides a boost function. When charging, the Inductor acts as a load and stores electrical energy; when discharging, it acts as a power source. The voltage developed during discharge is related to the current rate of change, independent of the original charging voltage, thus providing different input and output levels.
A boost regulator consists of two switches, two capacitors, and an 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 shoot-through currents. During phase 1 (tON), switch B is open and switch A is closed. The ON inductor is connected to ground, so current flows from VIN to ground. Since there is a positive voltage across the inductor, the current increases, allowing energy to be stored in the inductor. In phase 2 (tOFF), switch A is open and switch B is closed. The inductor is connected to the load, so current flows from VIN to the load. Since the inductor terminal is at a negative voltage, 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 the switching regulator can work either continuously or discontinuously in continuous conduction mode (CCM), in which the inductor current does not drop to 0; in discontinuous conduction mode (DCM), the inductor current in can be reduced to 0. The current ripple, shown as ΔIL in Figure 2, is calculated using the formula ΔIL = (VIN − tON)/L. The average inductor current flows into the load, while the ripple current flows into the output capacitor.
Figure 3. Boost Regulator Integrated Oscillator, PWM Control Loop, and Switching FET
A regulator that uses a Schottky diode instead of switch B is defined as an asynchronous (or non-synchronous), regulator, while a regulator that uses a FET as switch B is defined as a synchronous regulator. In Figure 3, switches A and B have been implemented using internal NFETs and external Schottky diodes, respectively, forming an asynchronous boost regulator. For low-power applications requiring load isolation and low shutdown current, external FETs can be added, as shown in Figure 4. Driving the device’s EN pin below 0.3 V shuts down the regulator, completely disconnecting the input from the output.
Figure 4. ADP1612/ADP1613 Typical Application Circuit
Modern low-power synchronous buck regulators use pulse-width modulation (PWM) as their primary mode of operation. The PWM keeps the frequency constant and adjusts the output voltage by changing the pulse width (tON).Average power delivered is proportional to duty D, so this is an efficient way to supply power to the load
For example, when the desired output voltage is 15 V and the available input voltage is 5 V:
D = (15 C 5)/15 = 0.67 or 67%.
Due to the reduced power dissipation, the input power must be equal to the power delivered to the load minus any losses. Assuming the conversion is very efficient, a small amount of power loss can be omitted from the basic power consumption calculation. Therefore, the input current can be approximately expressed as:
For example, if the load current is 300 mA at 15 V, then IIN = 900 mA at 5 V at 5 V—three times the output current. Therefore, the available load current decreases as the boost voltage increases.
The boost converter uses voltage or current feedback to regulate the selected output voltage; the control loop maintains output regulation as the load changes. Low-power boost converters typically operate in the frequency range of 600 kHz to 2 MHz. At higher switching frequencies, smaller inductors can be used, but doubling the switching frequency reduces efficiency by about 2%. In the ADP1612 and ADP1613 boost converters (see appendix), the switching frequency is pin-selectable and operates at 650 kHz for very high efficiency and 1.3 MHz for very small external components. For 650 kHz operation, connect FREQ to GND, and for 1.3 MHz operation, connect to VIN.
The inductor is the key component of the boost regulator, which stores energy during the on-time of the power switch and transfers it to the output through the output rectifier during the off-time. To balance low inductor current ripple with high efficiency, the ADP1612/ADP1613 data sheet recommends an inductor value range of 4.7 μH to 22 μH. In general, lower value inductors have higher saturation current and lower series resistance for a given physical size, and lower inductance results in higher peak current, which reduces efficiency and increases ripple and noise. It is usually good to perform the boost in discontinuous conduction mode to reduce inductor size and improve stability. The peak inductor current (high input current plus half the inductor ripple current) must be less than the inductor’s saturation current rating; and the regulator’s high DC input current must be less than the inductor’s rms current rating.
Boost Regulator Key Specifications and Definitions
Input voltage range: The input voltage range of the boost converter determines the very low input power available. Specifications may provide a wide input voltage range, but the input voltage must be lower than VOUT for efficient operation.
Ground Current or Quiescent Current: The DC bias current (Iq) not delivered to the load. Lower Iq results in higher 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 boost regulator for an application, it is good 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 stand by for long periods of time in sleep mode.
Switching duty cycle: The working duty cycle must be less than a large duty cycle, otherwise the output voltage cannot be adjusted. For example, D = (VOUT C VIN)/VOUT. When VIN = 5 V and VOUT = 15 V, D = 67%. The ADP1612 and ADP1613 have a large duty cycle of 90%.
Output Voltage Range: The output voltage range that the device can support. The output voltage of the boost converter can be fixed or can be adjusted using a resistor to set the desired output voltage.
Current Limit: Boost converters typically specify a peak current limit rather than load current. Note that the greater the difference between VIN and VOUT, the lower the available load current. Peak current limit, input voltage, output voltage, switching frequency, and inductor value all determine the maximum available output current.
Line Regulation: Line regulation refers to the rate of change of the output voltage as the input voltage changes.
Load Regulation: Load regulation refers to the rate of change of the output voltage as the output current changes.
Soft-start: It is important for the boost converter to have a soft-start function. The output voltage ramps up in a controllable manner during startup to avoid output voltage overshoot during startup. The soft-start of some boost converters can be adjusted with an external capacitor. As the soft-start capacitor charges, it limits the peak current allowed by the device. Start-up time can be varied to meet system requirements with an adjustable soft-start feature.
Thermal Shutdown (TSD): Thermal shutdown circuitry shuts down the regulator when the junction temperature exceeds specified limits. Consistently high junction temperatures can be caused by high operating current, poor cooling, or high ambient temperature.Protection circuitry includes hysteresis to prevent the device from returning to normal operation after the on-chip temperature drops below a preset limit after thermal shutdown
Under-Voltage Lockout (UVLO): If the input voltage falls below the UVLO threshold, the IC automatically turns off the power switch and enters a low-power mode. This prevents unstable operation that can occur at low input voltages and prevents the power device from starting up when the Circuit cannot control it.
The low-power boost regulator simplifies the design of the switch DC-DC converter by providing a proven solution. Design calculations are provided in the application section of the data sheet, and the ADIsimPower4 design tool simplifies the task for the end user.