Squeezing the Most From Battery Cells with a Switched-Mode Pump
Having an on-chip switched-mode pump in microcontrollers and SoCs is helpful in powering low-power embedded applications.
The typical operating voltage required for any microcontroller is at least 3.3 Volts. However, for the core in any microcontroller to be functional 1.8 Volts is sufficient. Since an AA or AAA battery will give 1.3-1.5V when fully charged, systems need at least two battery cells to operate. As the battery decay gradually falls below 0.9V, it becomes impossible to operate the system even with two batteries.
With the use of a boost converter, however, a microcontroller can operate on a single battery cell by boosting it up to 1.8V or higher. This allows systems to operate from one battery cell, and operate even when the voltage drops to 0.5V. Alternatively, systems can be powered with a single 0.5V solar cell. Developers can also protect system data when voltages drop too low to boost by employing techniques such as entering low power modes with RAM retention. This allows users to replace the battery and resume operation with no interruption. This article will explore how to use a switched-mode pump to solve these system power issues.
Achieving long battery life in portable applications is a challenge. Designers have multiple factors they have to consider to minimize power consumption, including power supply design, component selection, efficient firmware structure (if any), managing multiple low-power operating modes, and PCB Layout design. Many of today’s microcontroller and SoC architectures have an on-chip boost converter that can accept an input voltage supplied by a battery or other source and produce a selectable, higher output voltage than the input voltage.
Consider an application that uses a solar cell for power. A single Solar cell provides about 0.5V. Hence, we need at least three solar cells connected in series to generate a usable voltage. Typically solar cells are used in applications that are consumer-oriented and require a small form factor. With the availability of a boost converter, it is possible to power a microcontroller-based system with a single solar cell. After reading this article, a reader will be aware of the various features and modes of operation of a boost converter or switched-mode pump (SMP), problems faced when using an SMP, and techniques to improve efficiency.
Squeezing the battery
Figure 1: Discharge graph of a AA battery
Figure 1 shows the discharge graph of an AA battery of 2500 mAh capacity. Consider an application which consists of a controller or SOC that operates at 1.8 Volts and consumes an average current of 10 mA. The battery is expected to last for (2500 mA/10 mA) 250 hours. From the graph, we can see that once the battery voltage drops to 0.9 Volts, it has discharged about 2200 mAh. Beyond this point, even with two batteries (assuming the microcontroller works at 1.8V), all the features available in the controller may not operate normally. This means that the remaining 300 Ah, or more than 10%, of battery power cannot be used.
With a switched-mode pump available on the microcontroller, the battery voltage can be boosted to any usable voltage. Microcontroller manufacturers provide an option to select this usable voltage, allowing the voltage to be boosted to 1.8 Volts or higher, even when battery voltage drops below one volt, enabling the application to continue operating. By doing this, the system is able to extract a part of the remaining 300 mAh capacity still available in the battery cell.
Below a certain input voltage, the boost circuitry may not be able to operate, thus preventing the system from extracting all of the remnant power. Also note that the battery must be able to source enough current for the boost to function. The input current to the boost circuitry is a function of the input battery voltage and the output boosted voltage. This current increases as the difference between the input voltage and the output voltage increases; that is, as the battery voltage drops.
For example, consider an SMP being used to boost to a constant 3V output. In any system, the power is always constant (i.e., output power is equal to the input power). The output power from a boost converter, however, is slightly lower than the input power due to losses in the components used for conversion. Let us assume an ideal boost system with no loss. With the initial 1.5V battery input being boosted to 3V, to supply 50 mA to a load the input current would be (3*50)/1.5=100 mA. Once the battery voltage drops to one Volt, in order to maintain the output voltage the input current increases (power is constant). The input current in this case will be (3*50)/1=150 mA. Thus, the boost converter provides a constant voltage output regulation.
Figure 2: The architecture of a switched-mode pump (SMP) boost converter in an SoC
Figure 2 shows a boost converter circuit in comparison with the switched-mode pump in a microcontroller. VBat is the input battery voltage. Vsw is the Switching waveform, which is nothing but a PWM with a 50% duty cycle. The boost converter has two phases: a storage phase (switch is ON) and discharge phase (switch is OFF). When the switch is conducting, the inductor stores energy from the battery in the form of a magnetic field. When the switch is opened, the inductor current continues to flow in the same direction, and this causes the voltage at node Vsmp to “flyback” to a voltage higher than the capacitor voltage. This triggers the diode to begin conducting, which in turn allows the charge stored in the inductor to be transferred into the filter capacitor. The switch is turned On and Off by the PWM. In a microcontroller, this switching waveform is made available by a generation unit that is on-chip. The protection diode can be available internal to the microcontroller chip or it can be connected externally. The only components that a developer has to connect are the inductor coil and filter capacitors. In the SoC shown in the figure above, Vdda and Vddd are the chip supply voltage.
Design tips to improve the efficiency of an SMP
In a low power, low-input-voltage switched mode pump used in embedded solutions where space and cost are constrained, it is desirable to have very high efficiency. Efficiency is limited by the losses in the passive components. In this case, the MOSFET switch which is internal to the controller contributes to ohmic losses as well as switching losses. The higher the switching frequency, the greater the switching losses. The impedance of this switch is pretty much determined at the design stage of the chip. The inductor losses are similar to that of the switch. The switching frequency has to be chosen appropriately to optimize power and an inductor has to be chosen based on the switching frequency. The output capacitor can cause significant ripple due to its Equivalent Series Resistance (ESR). If aluminum capacitors are chosen to reduce cost, a ceramic capacitor should also be connected in parallel in order to minimize ripple. The hold time of the output voltage is determined by the size of the capacitor used. Schottky diodes are recommended because they have a low forward voltage and fast switching speed. The current rating of the diode should be greater than twice the peak load current. The Schottky diode also accounts for some amount of loss because of the forward drop and its own impedance.
The SMP in Figure 2 is shown to have an internal diode. In microcontrollers, however, this diode is mimicked using a MOSFET switch which is operated in synchronization with the switch. Having an external Schottky diode results in higher power loss, attributing to the diode drop, which typically would be typically be around 0.4 volt. The internal synchronous FET has lower drop (0.1 volt), as a result reducing losses for better efficiency. The efficiency of the SMP is also affected by the nature of the load. The efficiency is reduced if the load is not a constant load.
In a low input voltage SMP circuit, the layout design has to be done with extreme care. Consider the case of a boost convertor which starts up at 0.5v such as happens with Cypress's PSoC3 device. Let us assume that the Boost output is expected to be 3Volts with 50 mA current capability. With 100% efficiency, the input current is expected to be ((3*50)/0.5)=300mA. With 300mA current being pumped in, a PCB trace of 1ohm can easily produce a voltage drop of 0.3 Volt. Though the actual input voltage is about 0.5 Volt, what appears at the boost input would be 0.2 volt. As a result, the SMP would not start at 0.5 Volt input. This can be avoided by having wider traces of small length. The components have to be placed such that the conduction paths are kept short.
Another design issue is emissions due to the switching current into the SMP. When the inductor stores charge, the input current is higher. In addition, this current switches between two extremes as the inductor stores and discharges power. Consider a scenario where 0.5 volt is being boosted to about 3 volts. Assume the load draws about 50mA current. The input current in this case for an ideal SMP would be 300 mA. If the converter is not ideal, this current will be even higher. If there are any long traces through which this current flows, it may result in electromagnetic emissions which can affect the operation of other nearby circuits. If there any analog components nearby, for example, their performance might not be acceptable. This can be avoided by isolating the switching path from the other sensitive components by having guard traces that are connected to ground.
Additional Boost Converter features
The boost converter may also be used in any system that requires a higher operating voltage than the supply provides. For instance, this includes driving a 5.0V LCD glass in a 3.3V system. Also consider an example application having a controller and an RF chip for wireless communication (Figure 3). The RF chip may require 3.3 Volts for its operation while 1.8volts for the controller may be sufficient. In this scenario, an input regulated voltage can power the controller. At the same time, the SMP available on the controller can boost the input voltage to 3.3 Volts and supply the RF chip. This way the SMP on a controller can be used in applications that require multiple supplies.
Figure 3:MCU and RF chip for wireless communication
There are SoC manufacturers who make SoCs that have on-chip SMP with many unique features. Cypress Semiconductor, for example, offers the PSoC (Programmable System on Chip) architecture which has an SMP in addition to other resources such as precision programmable analog and digital components. The boost converter on a PSoC can be operated in two different modes: active and standby. Active mode is the normal mode of operation where the boost regulator actively generates a regulated output voltage from the battery input. In standby mode, most boost functions are disabled, thus reducing power consumption of the boost circuit. The converter can be configured to provide low power, low current regulation in standby mode. An external 32 kHz crystal can be used to generate inductor boost pulses on the rising and falling edge of the internal clock when the output voltage is less than the programmed value. This is called automatic thump mode (ATM). The boost typically draws 200 µA in active mode and 12 µA in standby mode. The switching frequency can be set to 100 kHz, 400 kHz, 2 MHz, or 32 kHz to optimize efficiency and component cost. The 100 kHz, 400 kHz, and 2 MHz switching frequencies are generated using oscillators internal to the boost converter block. When the 32 kHz switching frequency is selected, the clock is derived from a 32 kHz external crystal oscillator. The 32 kHz external clock is primarily intended for boost standby mode.
Having an on-chip Switched Mode Pump in microcontrollers and SoCs is helpful in powering low power embedded applications. Improving its efficiency helps improve the endurance of the battery. It also results in lesser number of batteries disposed, encourages designers to develop solar cell powered systems, and contributes towards a greener planet.
- PSoC® 1 Switch Mode Pump (SMP) Application Note
- PSoC®1: Family Data Sheet
- PSoC®3: Family Data Sheet
About the Author
Udayan Umapathi is an Applications Engineer with Cypress. He completed his Bachelor of Engineering in Electronics and Communication from RV College of Engineering, Bangalore. His expertise includes circuit design, embedded system design, and microcontrollers.