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Challenges with Measuring Current when Developing Power Management Schemes for Battery-Powered Devices (Part 2)

There are a number of solutions that can be used to make dynamic current measurements. Each carries with it a tradeoff of cost, simplicity, speed, and accuracy.

By Bob Zollo, Agilent Technologies, Inc.

scope

Part one of this two-part series covered the challenges of measuring current when developing power management schemes for battery-powered devices. To review, with battery life being a critical specification on portable devices, sophisticated power management schemes are being employed. In order to optimize power consumption and maximize battery life, engineers typically utilize advanced power-management schemes that rapidly turn on/off sub-circuits as necessary, creating dynamic current consumption typically from uA to A.

Unfortunately, traditional test instruments are not well suited for this type of measurement requirement. Digital multimeters (DMMs) can accurately measure over the dynamic range, for example, but they cannot measure rapid changes over time. Oscilloscopes can measure rapid changes over time, but due to the need for a current probe or shunt they lack accuracy (not enough vertical resolution) and dynamic range.

Are SMUs a solution?

The inability of traditional test instruments to make dynamic current measurements has, in many cases, led designers of battery-powered devices to try to use source/measure units (SMUs) for powering their designs and for measuring the resultant current. SMUs currently on the market are well known for their high accuracy in both sourcing and measurement. Therefore, while they may seem like a logical choice for sourcing power and accurately measuring currents drawn by battery-powered devices, this is not always the case. They are not ideal for dealing with the dynamic conditions present when sourcing power to, and measuring the power consumption of, a battery-powered device or one of its components. Conventional SMUs lack the voltage transient response necessary to hold a stable output. And, despite the fact that they offer very accurate measurements in a fixed range, they lack the ability to change ranges quickly and without disrupting the measurements, which is required for measuring dynamic currents.

A new and better solution for battery-powered device testing

Traditionally, SMUs lacked the capabilities necessary to adequately measure the dynamic currents associated with these methods. However, in June 2010, Agilent Technologies released two new SMUs with features specifically designed to address the needs of battery-powered device designers (Figure 1). These two SMUs are called the Agilent N6781A 2-Quadrant SMU for Battery Drain Analysis and the Agilent N6782A 2-Quadrant SMU for Functional Test. The new SMUs offer advanced sourcing and measurement capabilities required for the testing challenges associated with battery-powered devices and their components (Figure 2).

Agilent N66705B DC power analyzer
Figure 1: The Agilent N6781A 2-Quadrant SMU Module for Battery Drain Analysis
and the N6782A 2-Quadrant SMU Module for Functional Test work with the
Agilent N6705B DC Power Analyzer to create an R&D tool for battery-powered
device testing.

Agilent table
Figure 2: New Agilent SMU features and how they can help the engineer
to test battery-powered devices.

Most importantly - Measuring Dynamic Current

Sophisticated power management schemes create currents that are time-varying, with sub-millisecond events and wide dynamic ranges. There are a number of solutions on the market today that can be used to make dynamic current measurements. Each carries with it a tradeoff of cost, simplicity, speed, and accuracy.

However, the Agilent N6781A and N6782A eliminate the challenges of measuring dynamic currents with a feature called seamless measurement ranging. With seamless measurement ranging, engineers can precisely measure dynamic currents without any glitches or disruptions to the measurement. As the current drawn by the DUT changes, the Agilent SMU automatically detects the change and switches to the current measurement range that will return the most precise measurement. However, unlike autoranging available in DMMs or SMUs on the market today, there is no lag time while the instrument hunts for the correct range. With seamless measurement ranging, there is no loss of data or interruption of the measurement as it looks for and changes range.

When combined with the Agilent SMUs built-in 18-bit digitizer, seamless measurement ranging enables unprecedented effective vertical resolution of 28-bits. This enables engineers to see the complete current waveform they have never seen before, from nano-amperes to amperes, in one pass and one picture. Thus, it gives the engineer what they need to measure and visualize dynamic currents.

Battery Drain Analysis

To assess runtime, engineers use battery drain analysis, which entails characterizing the device, its firmware/software, and its sub-circuits, both independently and in combination as a system. Analysis techniques include characterizing the battery current drain and how it is affected by the various operating modes and use profiles. With this analysis, engineers can make power management design tradeoffs that maximize battery life. The Agilent N6781A 2-Quadrant SMU for Battery Drain Analysis offers the features required to simulate the battery and to accurately capture, thanks to seamless measurement ranging, the power consumption of portable, battery-powered devices. When used in conjunction with the new Agilent 14585A Control and Analysis Software, the N6781A becomes an even more powerful battery drain analysis solution offering greater measurement insight. Using the Agilent 14585A software, engineers can log measurements directly to the PC, visualize the data in scope or strip chart format, and perform statistical analysis of power consumption.

Functional Test

While battery drain analysis is a method for determining runtime of the battery, within a battery-powered device, there are power management components that need to be functionally tested to ensure they properly implement the intended power management scheme. For example, a cellular handset employs a circuit called a power management unit (PMU) or power management IC (PMIC). The job of these devices is to take the battery power and distribute it to sub-circuits within the handset. Some PMUs have one input (the battery voltage) and up to 20 outputs, with each output acting as an optimized voltage rail to its corresponding sub-circuit. So, to functionally test the PMU, the engineer would need a voltage source (to simulate the battery) and use many small electronic loads to load down the outputs of the PMU.

For dynamic testing, such as measuring if disturbances on the input carry through the PMU and become disturbances on the output, a fast dc source is needed to simulate the battery. If the engineer is interested in knowing if disturbances on one PMU output channel impact the other channels, then a dc load that can provide time varying load current transients is desired. Ultimately, a single picture view of the inputs voltage and current and the output voltage and current is desirable, correlated in time if the engineer wants to see how disturbances one channel impact another.

The Agilent N6782A 2-Quadrant SMU for Functional Test can modulate its output up to 100 kHz and provides 2-quadrant operation. This makes the N6782A a great fit for advanced functional test for a variety of devices including DC/DC converters, power management units, power amplifiers and power management ICs. The input stage of the DUT can be stimulated by the N6782As fast sourcing and waveform capabilities. The output stage can be loaded down and measured with the electronic load capabilities of the Agilent N6782A. Being able to stimulate the input of a DUT, as well as load down the output, provides a total test solution. The Agilent 14585A Control and Analysis Software aids in functional test by allowing easy creation of complex waveforms to stimulate or load down a DUT by inputting a formula, choosing from built-in, importing waveform data, or recording an event (using the N6782As scope mode) and playing it back (using the N6782As arbitrary waveform generator). The system can also log measurements directly to the PC and perform statistical analysis of power consumption.

Summary

With battery life being a critical specification on portable devices, sophisticated power management schemes are being employed. The result is that the currents being drawn by these devices are time-varying, with sub-millisecond events and wide dynamic ranges. Engineers will need to characterize these currents in order to determine if they have achieved improved optimization of available battery power.

There are a number of solutions that can be used to make dynamic current measurements. Each carries with it a tradeoff of cost, simplicity, speed, and accuracy. Agilent Technologies offers the N6781A 2-Quadrant SMU for Battery Drain Analysis, the N6782A 2-Quadrant SMU for Functional Test, and the 14585A Control and Analysis software. These products are specifically designed to overcome the shortfalls of existing instrumentation and allow engineers to source power without transient droop as if it was coming from a battery and to measure rapidly changing dynamic currents, from nA to A, in order to visualize current waveforms being drawn by the battery-powered device.



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