Battery consumption analysis for mobile devices

Battery life is an important factor in the design of mobile devices. Many mobile devices have added more features, and these new features will quickly reduce run time. Engineers must take advantage of complex power management schemes to get the longest battery life.

Engineers need to use battery consumption analysis to evaluate battery run time. This analysis requires the device, firmware/software and its subcircuits to be described separately and separately in the system. Analytical techniques include describing 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 save battery power by putting subsystems that are not actively active into sleep on a sub-millisecond time scale. As a result, the device has a rapidly changing current in an on/off event that occurs within less than 1 s. For example, a GSM handset can have a 560 μs, 2 A pulse when transmitting, and then when in standby mode, the current level may drop to milliamps during the sleep cycle.

Verify battery time

One way to verify battery operating time is to use a voltage drop test that uses a fully charged battery to power the device under test (DUT) in standby mode until the battery is dead. This test can be relatively time consuming because it requires the completion of all process runs to determine the voltage shutdown point to determine the operating time. Again, the results depend on the initial state of the battery, which can vary widely.

Another method is to perform current consumption measurements, which provide greater confidence in working time measurements. The DUT is placed in the operating mode to be evaluated for a short period of time and measures the current consumption in this particular mode of operation. The working time is then calculated by dividing the nominal battery capacity by the measured current draw. Using this method, the designer can determine the run time without waiting for the battery to fully discharge.

Part of an ideal system

In an ideal system for performing battery consumption analysis (as shown in Figure 1), the first element required is a method of placing the DUT in an appropriate mode of operation for target testing (DUT excitation). For mobile phones, a base station emulator is typically used.

Figure 1. Several components present in a general ideal system for battery current consumption measurement and analysis

Second, you need a proper DUT power supply method, using a battery or power supply. The purpose of the 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 these states (full charge, partial discharge, full discharge / end of life).

Other important system components are: current transformers for measuring current, digitizers for recording voltage and current signals, and software for analyzing and storing test data. These test data can be very large for long-term testing. Can be up to several gigabytes.

Measurement considerations

The power supply used in the battery drain analysis must describe the DUT independently of the battery. The power supply must have a fast response to minimize the transient voltage drop caused by the fast swing 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 emulated power supply) that can tolerate these conditions without voltage drop should be used.

The rapidly changing current waveforms from the battery to the mobile device present two measurement challenges: range and speed. First, the dynamic range of the current may exceed 1000:1 or even 1 000 000:1. The full-power active current is on the order of 1-3A, while the low-sleep mode current is on the order of tens of microamps, so the range of current to be tested presents a challenge for the choice of current converter.

Current-aware resistors or shunts can be used here, but choosing a properly sized shunt can be tricky. If the size of the shunt is suitable for measuring the minimum current, then a large voltage drop will occur across the shunt in the event of a large current, which will impose an unacceptable voltage burden on the circuit. If the size of the shunt is suitable for measuring large currents, then when the microampere current flows, there is a high probability that there is not enough voltage available for measurement. By having several shunts for different current magnitude measurements, engineers can solve signal level problems, but switching shunts means interrupting measurements.

In terms of measurement speed, digitizers used to measure current shunt voltage and mobile device bias voltage should have a sampling rate of 50 kHz or faster to capture sub-millisecond pulses, which are characteristic of complex power management schemes.

Simplify complex analysis

Communication systems such as 3G employ complex modulation formats characterized by high-order amplitude modulation required to transmit higher data rates. From the time domain, the resulting current consumption waveform is complex and random.

When operating for a long time and performing different operations, the current consumption-time diagram (shown in Figure 2) of the RF power amplifier of a cdma2000 mobile phone using three data channels is complicated and unpredictable. This is common for battery life testing and it is difficult to observe the effect of changing the current consumption design.

Figure 2 From the time domain (left), the current consumption waveform of the RF power amplifier of the cdma2000 phone is complex and unpredictable. By looking at the same current waveform in CCDF plot (b), the designer can easily see how often the device is in each current state.

A better way to visualize and analyze complex current consumption patterns is to use a Complementary Cumulative Distribution Function (CCDF) map to view their statistical distribution. In the CCDF diagram, the x-axis represents the current and the y-axis represents the cumulative percentage of its occurrence (as shown in Figure 2b).

By looking at the statistical distribution of the magnitude of the current drawn, designers can quickly see how often the device is operating in each current state. For different design schemes, comparing these CCDF graphs, you can see when the device consumes more energy (that is, the proportion of its time increases under high current conditions), or when it consumes less energy (that is, Said that the proportion of its time increases in the small current state). As a result, engineers can assess when the design is better (requiring less energy) or identifying design flaws (expectingly requiring more energy).

Ready-made solution

Products from several test equipment suppliers can handle different parts of the target test system. Some vendors offer a power supply that provides a stable, battery-like output when pumping fast current pulses.

The entry-level solution is Agilent Technologies' mobile communications DC source 66300 series. This series is specially tailored to power mobile devices while measuring their current consumption. It combines a battery emulation power supply with a high-speed digital measurement system similar to an oscilloscope to provide accurate current measurements for the device's active, standby, and shutdown modes.

This DC source and its associated turnkey software allow users to see their current waveforms in an oscilloscope-like view, data logger view and on a CCDF chart without any programming. If you have higher requirements for accuracy and sampling rate, you can also choose other solutions offered by the company.

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