As both businesses and individuals become increasingly dependent on (or some might say, addicted to) mobile communication products, the portion of integrated circuits and electronic sub-assemblies being designed for use in these applications is growing rapidly. Being mobile means these devices typically operate on battery power, so they have a limited supply of power available to them. To achieve their battery operating life targets, designers must be able to minimize the power consumption of their devices. This demands the ability to characterize the power consumption of the device in all states to understand where to focus their efforts.
One of the biggest challenges with characterizing the power consumption of mobile devices is that their current draw can vary greatly, depending on the state in which the device is operating. Mobile devices often spend a majority of their time in sleep mode, drawing only microamps of current or less, only waking up periodically to perform their function. While performing these functions, current draw can increase dramatically, jumping to tens, hundreds or even thousands of milliamps. In many cases, this increased current draw lasts for as little as a few hundred microseconds, which makes measuring the device’s current consumption during these operations difficult.
Unfortunately for mobile device developers, conventional power supplies lack the ability to measure the sudden current pulses drawn by a device while it is awake, so they need to integrate additional instrumentation into their test systems, which not only increases the complexity of their test setup and complicates triggering but adds another item to their test equipment budget. However, an emerging class of DC power supplies with integrated precision measurement capabilities offers these developers the ability to measure ultra-short pulse currents as well as the triggering features required to synchronize the measurement with the current pulse.
This article describes how one of these instruments was used to measure the pulse of load current drawn by an 802.15.4 Wireless Transceiver Module during its data transmission state. This transceiver is commonly used with the ZigBee network stack for creating products designed for building automation and remote sensor monitoring. The transceiver used includes an SPI digital interface for configuration and operation. This interface allows the device to easily be controlled by a microcontroller. However, for the purposes of testing the device, a USB-based SPI/I2C controller with a digital I/O port was used to configure and operate the device from test code running on a PC. This test code also configured and controlled a Keithley Model 2280S Precision Power Supply, which supplied power to the DUT and made the pulsed current measurements.
The VCC and GND pins of the wireless transceiver were connected to the instrument’s Hi and Lo leads. To ensure an accurate voltage was applied at the device, the sense lead jumpers were removed from the connector on the rear of the instrument and a separate set of sense leads were run to the VCC and GND pins of the device. This allowed the instrument to compensate for any voltage drops in Hi and Lo leads as a result of the device’s large current draw during transmission.
The Chip Select ( (CS) ̅ ), MISO, MOSI and SCLK lines of the wireless transceiver module’s SPI interface were routed to the USB-to-SPI/I2C controller’s SPI lines. The Reset, Interrupt, and Sleep/Transmit lines were routed to the USB-to-SPI/I2C controller’s digital I/O port. The Sleep/Transmit line and GND lines were also routed the instrument’s digital I/O port. Toggling this line offers a way to control data transmission for this wireless transceiver, which makes it a useful trigger source for a measurement. Note that this line couldn’t be used directly to trigger the instrument, so an in-line converter was added.
Although the wireless transceiver module, along with all of its digital signals, operates at 1.8V, the instrument’s digital I/O port requires at least 3.4V, so it’s not high enough to register as a signal level Hi on the instrument’s digital I/O; as a result, it cannot be used directly to trigger the instrument. Furthermore, the signal that starts the transmission on the wireless transceiver is a rising edge; however, a falling edge is required to trigger the instrument. Therefore, the line must first be level-converted in order to use the Sleep/Transmit line of the wireless transceiver as a trigger signal.
Figure 2 illustrates how a couple of resistors and a small signal NPN BJT (Part# 2N3904) were used to create a simple circuit that converts the signal from 1.8V to 5V and inverts the polarity of the signal, creating a 5V falling edge trigger from the 1.8V rising edge trigger.
To allow the instrument to characterize high speed current pulses accurately, it was configured for fast, synchronized operation. This included turning off the averaging filter, choosing the instrument’s fastest measurement speed, switching off auto-zeroing, etc. This allowed the instrument to make a fast current measurement every time it received a trigger on its digital I/O port. Using the DUT’s Sleep/Transmit line as a trigger source, the measurement was aligned with the top of the current pulse. The measurement and data collection sequence was automated using SCPI commands sent from the PC. Figure 3 illustrates the sequence.
The instrument readings in Table 1 are from 10 separate current pulses in which the same data was transmitted each time. Note the high precision with which the current can be measured, with up to 6-1/2 digits of resolution. Averaging can be used for even greater precision.
Sample Current
0 14.552E-3
1 14.854E-3
2 15.189E-3
3 15.156E-3
4 14.955E-3
5 14.921E-3
6 14.921E-3
7 15.022E-3
8 15.055E-3
9 15.022E-3
Average 14.965E-3
Table 1. Readings from 10 separate current pulses
To learn more about characterizing mobile device power consumption to achieve battery operating life target, download a free copy of Keithley’s application note, “Making Pulse Current Measurements with the Series 2280S Precision Measurement DC Power Supplies.”
# # #
David Wyban is an applications engineer with Keithley Instruments, Inc., Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio. He holds a bachelor’s degree in electrical and computer engineering from The Ohio State University. He can be reached at dwyban@keithley.com.