Application of DC-DC Boost Regulator in Battery Powered System

Portable electronic devices such as smartphones, GPS navigation systems, and tablets can be powered by low-voltage solar panels, batteries, or AC-DC power supplies. 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 the magnetic field of an inductor to alternately store energy and release it to the load at different voltages. Because the losses are very low, they are a good and efficient choice. Capacitors connected to the output of the converter reduce the output voltage ripple. The boost converter discussed in this article provides a higher voltage, while the buck converter discussed previously provides a lower output voltage. Switching converters with built-in FETs as switches are called switching regulators, and switching converters that require external FETs are called switching controllers.

Figure 1 shows a typical low-power system powered by two AA batteries in series. The available output range is approximately 1.8 V to 3.4 V, while the IC requires 1.8 V and 5.0 V to operate. A boost converter can increase the voltage without increasing the number of battery cells to power WLED backlights, micro hard drives, audio devices, and USB peripherals, while a buck converter can power microprocessors, memory, and displays.

Figure 1. Typical low-power portable system

The inductor's tendency to oppose changes in current provides a voltage boost function. When charging, the inductor acts as a load and stores energy; when discharging, it can act as a source. The voltage generated during the discharge process is related to the rate of change of the current and is independent of the original charge voltage, thus providing different input and output levels.

The 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 current. In the first phase (tON), switch B is open and switch A is closed. The inductor is connected to ground, so the current flows from VIN to ground. Since the inductor terminals are at a positive voltage, the current increases, causing energy to be stored in the inductor. In the second phase (tOFF), switch A is open and switch B is closed. The inductor is connected to the load, so the current flows from VIN to the load. Since the inductor terminals are at a negative voltage, the current decreases, and the energy stored in the inductor is released to the load.

Figure 2. Buck converter topology and operating waveforms

Note that a switching regulator can operate either continuously or discontinuously. In continuous conduction mode (CCM), the inductor current does not drop to zero; in discontinuous conduction mode (DCM), the inductor current can drop to zero. 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. A boost regulator integrates the oscillator, PWM control loop, and switching FET.

Regulators that use a Schottky diode in place of switch B are defined as asynchronous (or nonsynchronous) regulators, while regulators that use a FET as switch B are defined as synchronous regulators. In Figure 3, switches A and B have been implemented using an internal NFET and an external Schottky diode, respectively, to form an asynchronous boost regulator. For low-power applications that require load isolation and low shutdown current, an external FET can be added, as shown in Figure 4. The regulator can be shut down by driving the device’s EN pin below 0.3 V, 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 operating mode. PWM keeps the frequency constant and adjusts the output voltage by varying the pulse width (tON). The average power delivered is proportional to the duty cycle D, so this is an efficient way to deliver power to the load.

For example, when the required output voltage is 15 V and the available input voltage is 5 V:

D = (15 – 5)/15 = 0.67 or 67%.

Because power dissipation is reduced, the input power must equal the power delivered to the load minus any losses. Assuming very efficient conversion, small amounts of power losses can be omitted from basic power dissipation calculations. The input current can therefore be approximated by:

For example, if the load current is 300 mA at 15 V, then IIN = 900 mA at 5 V—three times the output current. Therefore, the available load current decreases as the boost voltage increases.

Boost converters use voltage or current feedback to regulate the selected output voltage; a control loop maintains output regulation as the load changes. Low-power boost converters typically operate in the 600 kHz to 2 MHz range. Higher switching frequencies allow smaller inductors to be used, but efficiency decreases by approximately 2% for every doubling of the switching frequency. In the ADP1612 and ADP1613 boost converters (see Appendix), the switching frequency is pin-selectable, with the highest efficiency at 650 kHz and the minimum external components at 1.3 MHz. For 650 kHz operation, connect FREQ to GND, and for 1.3 MHz operation, connect FREQ to VIN.

The inductor is a key component of the boost regulator, storing energy during the on-time of the power switch and transferring energy to the output through the output rectifier during the off-time. To strike a balance between low inductor current ripple and high efficiency, the ADP1612/ADP1613 data sheet recommends an inductor value range of 4.7 μH to 22 μH. Generally speaking, lower value inductors have higher saturation currents and lower series resistance for a given physical size, while lower inductance results in higher peak currents, which can reduce efficiency and increase ripple and noise. It is usually best to perform the boost in discontinuous conduction mode to reduce the inductor size and improve stability. The peak inductor current (maximum input current plus half of the inductor ripple current) must be less than the rated saturation current of the inductor; and the maximum DC input current of the regulator must be less than the current rms rating of the inductor.

Boost Regulator Key Specifications and Definitions

Input Voltage Range: The input voltage range of the boost converter determines the lowest usable input supply. The specifications may provide a wide input voltage range, but the input voltage must be lower than VOUT to achieve high efficiency operation.

Ground or Quiescent Current: The DC bias current (Iq) that is not delivered to the load. Lower Iq means higher efficiency, however, Iq can be specified for many conditions including shutdown, zero load, PFM mode of operation, or 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 stay in sleep mode for a long time.

Switching Duty Cycle: The operating duty cycle must be less than the maximum duty cycle, otherwise the output voltage cannot be regulated. For example, D = (VOUT – VIN)/VOUT. When VIN = 5 V and VOUT = 15 V, D = 67%. The maximum duty cycle of ADP1612 and ADP1613 is 90%.

Output Voltage Range: This is the range of output voltages that the device can support. The output voltage of a boost converter can be fixed or adjustable using resistors to set the desired output voltage.

Current Limit: Boost converters usually specify a peak current limit instead of a load current. Note that the greater the difference between VIN and VOUT, the lower the available load current. The 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 output voltage as the input voltage changes.

Load regulation: Load regulation refers to the rate of change of output voltage as output current changes.

Soft Start: It is important for boost converters to have a soft start feature, which ramps the output voltage up in a controlled 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 in the device. With an adjustable soft start feature, the startup time can be varied to meet system requirements.

Thermal Shutdown (TSD): Thermal shutdown circuitry 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, or high ambient temperature. The protection circuitry includes hysteresis to prevent the device from returning to normal operation after thermal shutdown occurs and until the on-chip temperature drops below a preset limit.

Undervoltage Lockout (UVLO): If the input voltage is below the UVLO threshold, the IC automatically turns off the power switch and enters low power mode. This prevents unstable operation that may occur at low input voltages and prevents the power device from starting when the circuit cannot control it.

Conclusion

Low-power boost regulators simplify the design of switching DC-DC converters by providing a mature design.