How to apply SMP to obtain greater battery storage capacity

　　Many of today's microcontroller and SoC architectures include an on-chip boost converter that accepts an input voltage from a battery or other power source and generates an output voltage that is selectably higher than the input.

　　Getting long battery life in portable applications is a difficult task. Designers who do power optimization must consider many factors, such as power supply design, component selection, efficient firmware structure (if any), management of multiple low-power operating modes, and PCB layout design. This article explores the use of SMP (switch mode pump) as a boost converter to solve the system power problem.

　　The typical operating voltage required by any microcontroller is at least 3.3V, of course, for its core, 1.8V is enough to work. AA or AAA batteries provide a voltage of 1.3V~1.5V when fully charged, so the system needs two batteries to work. Since the voltage will be lower than 0.9V when the battery is discharged, the system cannot run even with two batteries.

　　But with a boost converter, a microcontroller can boost the voltage of a single battery to 1.8V or higher. Not only does the boost converter allow the system to operate with a single battery, but it also allows the system to operate when the battery voltage drops to 0.5V. In addition, solar cell-powered devices (generally small consumer products) can also use the boost conversion method, so that a single 0.5V solar cell can operate instead of three 0.5V batteries. Developers can also use low-power mode techniques such as RAM maintenance (at this time, the user can replace the battery and the system will resume operation without interruption) to protect the system's data when the voltage is too low to do a boost.

　　Draining the battery

　　Figure 1 shows the discharge curve of a 2500 mAhr capacity AA battery. Consider an application that includes a controller or SoC operating at 1.8V and consuming an average of 10 mA. The expected battery life is 2500 mAhr/10 mA, or 250 hours. As shown in Figure 1, when the battery voltage drops to 0.9V, it has lost about 2200 mAhr of its capacity. Beyond this point, even with two batteries (assuming the microcontroller operates at 1.8V), the existing functions in the controller will not work properly. This means that the remaining 300mAhr (or 10% more) of the battery cannot be used.

　　If there is a switch-mode pump in the microcontroller, the battery voltage can be boosted to a suitable usable voltage. The microcontroller manufacturer provides an option to select the usable voltage, so that the voltage can be boosted to 1.8V or higher to power the application, even if the battery voltage drops below 1V. The system can then get some power from the battery that still has 300mAhr left.

　　However, below a certain input voltage, the boost circuit will not work, thus limiting the system from obtaining all the remaining energy. Note that the battery should be able to provide sufficient current for the boost operation. The input current of the boost circuit is a function of the input battery voltage and the output boost voltage. When the battery voltage drops, this current increases due to the increase in the difference between the input voltage and the output voltage.

　　For example, consider an SMP that is used to boost a voltage to a constant 3 V output. The power in any system is always constant, i.e. the output power equals the input power. The output power of a boost converter is slightly less than the input power because there are losses in the components used for the conversion, but we assume an ideal boost system with no losses. Initially, the 1.5 V battery input is boosted to 3 V to provide a 50 mA current to a load, and the input current is ((3×50)/1.5) mA = 100 mA. When the battery voltage drops to 1 V, the input current required to maintain the same output voltage increases (the power remains constant), and the input current is now ((3×50)/1) mA = 150 mA. In this way, the boost converter provides a constant output voltage regulation.

　　Architecture

　　Figure 2 shows a comparison of the circuit architecture of an SMP boost converter built into an SoC and an external boost converter. The boost converter shown in Figure 2a has two stages: a storage stage, when the switch is open, and a discharge stage, when the switch is closed. When the switch is on, the inductor stores energy from the battery in the form of a magnetic field. When the switch is off, the inductor continues to provide current in the same direction, causing the voltage at node VSMP to "fly back" to a voltage higher than the capacitor voltage. This action triggers the diode to begin conducting, allowing the charge stored in the inductor to be transferred to the filter capacitor. A PWMVSW is responsible for opening and closing the switch.

　　In a microcontroller (Figure 2b), an on-chip generator provides this switching waveform. The protection diode can be built into the microcontroller chip or can be external. The only components that the developer needs to connect are the inductor and filter capacitor. In the SoC shown in Figure 2b, VDDA and VDDD are the supply voltages of the chip.

　　Design Tips

　　The low-power, low-input voltage SMPs used in embedded solutions require high efficiency. Such applications are constrained by space and cost, but the losses of switching components and passive components will limit the improvement of efficiency. The MOSFET switch built into the controller will cause ohmic losses and switching losses; the higher the switching frequency, the greater the switching loss. The impedance of the switch is mainly determined in the design stage of the chip, and the inductor loss is similar to the switching loss. Designers must select the appropriate switching frequency to optimize power, and must select the inductor based on the switching frequency.

　　The ESR (equivalent series resistance) of the output capacitor can produce a large ripple. If an aluminum electrolytic capacitor is selected to reduce cost, a ceramic capacitor should also be connected in parallel to reduce the ripple. The size of the capacitor used determines the hold time of the output. Schottky diodes are recommended because they have a low forward voltage drop and high switching speed, but the forward voltage drop of the Schottky diode and its own impedance also cause some losses. The rated current of the diode should be greater than twice the peak load current.

　　The SMP in Figure 2b has an internal diode. However, in the microcontroller, this diode is simulated by a MOSFET switch, which works synchronously with the SMP. If an external Schottky diode is connected, it will cause higher power loss due to the forward voltage drop of the diode, which is generally about 0.4V. The built-in synchronous FET has a lower voltage drop (0.1V), thus minimizing losses and improving battery efficiency.

　　The load characteristics also affect the efficiency of the SMP; if it is not a constant load, the efficiency will decrease.

　　The layout design for a low-input voltage SMP circuit must be done very carefully. Consider a boost converter that starts at 0.5V, such as the PSoC3 (Reference 1) programmable single-chip system from Cypress Semiconductor. Let's assume that the boost output is expected to be 3V at 50mA. With 100% efficiency, the input current is expected to be ((3×50)/0.5)mA = 300mA. With 300mA pumped in, a 1Ω PCB trace can easily produce a 0.3V drop. Although the actual input voltage is about 0.5V, only 0.2V is left at the boost converter input. Therefore, the SMP cannot start with a 0.5V input voltage. Board designers can use some layout techniques to avoid this situation, such as using wider and shorter traces and placing components to keep the conductive paths as short as possible.

　　Another design issue is the radiation generated by the switching current flowing into the SMP. When the inductor is storing charge, the input current is high. In addition, as the inductor stores and releases energy, this current switches between two extremes.

　　Consider a case where the voltage is boosted from 0.5V to about 3V, assuming the load current is about 50mA. At this point, the input current to the ideal SMP is 300mA. If the converter is non-ideal, this current will be much higher. If this current passes through any length of trace, the electromagnetic radiation will affect the operation of the adjacent circuits. For example, if there are any analog components nearby, their performance may be affected. To avoid this situation, use a grounded guard trace to isolate the switch path from other sensitive components.

　　Characteristics of Boost Converter

　　Any system that requires a voltage higher than the supply voltage can also use a boost converter. An example is driving a 5V LCD from a 3.3V system.

　　For another example, an application has a controller and an RF chip for wireless communication (Figure 3). The RF chip may require 3.3V to operate, while the controller only needs 1.8V. In this case, the input regulated voltage can power the controller; at the same time, the SMP on the controller can boost the input voltage to 3.3V to power the RF chip. Therefore, the SMP on the controller can be used in applications that require multiple power supplies.

　　Many manufacturers offer SoCs with on-chip SMPs that have unique features. Cypress Semiconductor's PSOC architecture is an example, which includes an SMP in addition to other resources such as precision programmable analog and digital components. The boost converter on the SoC can operate in active or standby mode. Active mode is the normal operating mode, when the boost regulator obtains the battery input voltage and produces a regulated output voltage. In standby mode, most of the boost power is turned off to reduce the power of the boost circuit. The converter can be configured to provide low power and low current regulation in standby mode. When the output voltage is less than the set value, an external 32kHz crystal can be used to generate inductive boost pulses on the rising and falling edges of the internal clock. This mode is called ATM (automatic hammering mode).

　　The boost current is typically 200μA in active mode and 12μA in standby mode. The switching frequency can be set to 100kHz, 400kHz, 2MHz or 32kHz to optimize efficiency and component cost. The 100kHz, 400kHz and 2MHz switching frequencies are derived from the internal oscillator in the boost converter. When the 32kHz switching frequency is selected, the clock is derived from an external 32kHz crystal. The 32kHz external clock is primarily used in the boost standby mode.

　　On-chip SMPs in microcontrollers and SoCs help provide power for low-power embedded applications. They improve battery efficiency, increase battery life, and reduce the number of discarded batteries. SMPs also encourage designers to develop systems powered by solar cells.