How to analyze battery consumption in mobile devices

Battery run time is an important factor in mobile device design. Many mobile devices are adding more features that quickly reduce run time. Engineers must use sophisticated power management schemes to get the longest run time from the battery. Engineers need to use battery consumption analysis to evaluate battery run time, which requires characterizing the device, firmware/software, and its subcircuits both individually and integrated into the system . Analysis techniques include characterizing battery current consumption and how it is affected by various operating modes and usage profiles. With this analysis, engineers can make power management design trade-offs to maximize battery life. Most power management systems conserve battery energy by putting subsystems that are not actively in use to sleep on a sub-millisecond timescale. As a result, devices have rapidly changing currents during on/off events that occur in less than 1 second. For example, a GSM phone can have a 560μs, 2A pulse when transmitting, and then when in standby mode, the current level may drop to milliamps during sleep cycles. Verifying Battery Time One method of verifying battery run time is to use a voltage drop test, using a fully charged battery to power the device under test (DUT) in the operating mode to be verified until the battery is discharged. This test can be relatively time consuming because it requires the entire process to be run through to determine the voltage shutdown point to determine the run time. Again, the results are dependent on the initial state of the battery, which can vary widely. Another method is to perform a current drain measurement, which provides higher confidence in the run time measurement. The DUT is placed in the operating mode to be evaluated for a short period of time and the current drain in this specific operating mode is measured. The run time is then calculated by dividing the nominal battery capacity by the measured current drain. Using this method, the designer can determine the run time without waiting for the battery to fully discharge. Components of an Ideal System In an ideal system for performing battery drain analysis (as shown in Figure 1), the first element required is a method to put the DUT into the appropriate operating mode for the target test (DUT stimulus). For mobile phones, a base station simulator is typically used.
















Second, a proper method of powering the DUT is required, either with a battery or a power supply . The purpose of a power supply is to test the DUT independently of the battery to ensure consistent testing or to quickly replicate various battery states without waiting for the battery to reach those states (fully charged, partially discharged, fully discharged/end of life). Other important system components are current converters to measure the current, digitizers to record the voltage and current signals, and software to analyze and store the test data, which can be very large, running into several gigabytes, to complete long-term testing. Measurement Considerations The power supply used in battery drain analysis must characterize the DUT independently of the battery. The power supply must have a fast response to minimize transient voltage drops caused by the fast-slewing current pulses that the DUT has when switching modes or transmitting pulses. Many general-purpose power supplies can experience transient drops of up to 1V under these conditions, so a dedicated power supply (sometimes called a battery emulation power supply) that can tolerate these conditions without voltage drops should be used. The rapidly changing current waveform flowing from the battery to the mobile device presents two measurement challenges: range and speed. First, the dynamic range of current can exceed 1,000:1 or even 1,000,000:1. With full-power active currents in the order of 1 to 3A and low sleep-mode levels in the tens of microamps, the range of currents to be measured presents a challenge in selecting a current converter. Current-sensing resistors or shunts can be used here, but choosing the right shunt size can be tricky. If the shunt is sized for the smallest currents, a large voltage drop will appear across the shunt during a large current event, which will place an unacceptable voltage burden on the circuit. If the shunt is sized for large currents, there is a high probability that there will not be enough voltage to measure when microamps of current flow. Engineers can resolve the signal level issue by having several shunts for different current levels, but switching shunts means interrupting the measurement. In terms of measurement speed, the digitizer used to measure the current shunt voltage and the bias voltage of the mobile device should have a sampling rate of 50kHz or faster to capture the submillisecond pulses that are a feature of complex power management schemes.