As wireless mobile devices become more and more capable, the demands for longer battery
life continue to increase. Optimizing the operation and efficiency of the individual components
in a mobile device is key to improving its overall efficiency. This video will
focus on design verification testing of on-board DC to DC converters in mobile devices. The circuit
components shown here provide several different internal voltage levels from the single voltage
level provided by the battery. They are often incorporated into a single package known as
a Power Management Unit or Power Management Integrated Circuit. DC to DC converter efficiency
is a key parameter to optimize in the design of these components. The efficiency of the
DC to DC converter and all the components used in the design directly affect the operating
life of the battery-powered device. Efficiency must be measured over the entire range of
current required by the device in use and over the battery voltage range from full charge
to low charge. Because different circuits of the wireless device often switch between
various sleep modes and active modes, the turn-on time and rise time of the DC to DC
Converters are also key design parameters that affect efficiency. Minimizing the transition
time reduces battery drain when no productive activity is occurring in the device. Measuring
and controlling these turn-on times and rise times is a key part of improving device efficiency
and battery powered operating time. First, let’s look at efficiency measurements. A typical test configuration
using conventional test equipment would require a power supply, an electronic load, two external
current shunts and a four channel data logger for the measurements. In addition, the efficiency
of the converter must be calculated from the raw voltage and current measurements on the
input and output of the DC to DC converter. These measurements must be correlated in time
if you’re testing the converter under dynamic conditions. Thorough testing for efficiency
requires setting the power supply to voltages representing the battery voltage at full charge,
nominal level, and the drop-out or turn-off level. For each voltage input level to the
converter, the load on the converter’s output must be stepped or swept over the load current
range that is expected in actual device operation. This may range from micro-amps in sleep mode
to hundreds of milli-amps in transmit mode. Setting up the source and load manually and
making all these measurements with conventional equipment can be extremely time consuming.
As the load current is stepped or swept, a corresponding measurement of the input voltage,
input current, and output voltage must be captured and logged for analysis. Accurately
measuring these widely varying current levels will require changing the shunt values to
minimize the voltage drop across the shunt. This makes it difficult to automate the test
process and therefore the design verification task is more time consuming. There is a better
test solution from Agilent Technologies for making these challenging measurements faster
and with more accuracy. The Agilent N6782A Source/Measure Unit installed in a N6705B
DC Power Analyzer mainframe has specialized features for testing DC to DC converters used
in PMU’s. This is a modular system where up to four SMU’s can be installed in the
mainframe for maximum flexibility. Each SMU can be configured as a source or load and
includes voltage and current measurement capability. Diagrammed here is the setup for testing a single DC
to DC converter. When configured as a source, the N6782A SMU provides programmable voltages
with faster settling and less ringing under dynamic loads than a general purpose power
supply. When configured as a load the N6782A SMU has unique attributes well suited
for testing DC to DC Converters. With 2-quadrant operation, it can perform as a constant-voltage
or constant-current electronic load. The built-in arbitrary waveform generator makes it easy to create a wide variety
of dynamic loading profiles to apply to the converter. This example shows the SMU’s
load current changing from 500 microamps to 1 amp during a load sweep and then returning
to its minimum value. The N6782A SMU has an internal high-speed 200KHz digitizer with
seamless measurement ranging so you can accurately capture the entire dynamic range of measurements
in a single pass. This capability means that you can capture and view the dynamic voltage
and current consumption of the device under test with an effective resolution of up to
28 bits. The internal ammeter eliminates the need for external shunts. When combined
with the Agilent 14585A software, this solution has the ability to automate your test process
and greatly reduce the time spent in design verification of DC to DC converters.
Using the built-in ARB, you can create waveforms up to 100 kHz to
stimulate dynamic loads on a device under test. This screen shows the process for setting
up a swept load waveform. Here is an example of the results of an efficiency test on a
DC to DC converter. The load was swept from 500 micro-amps to one amp while the input
and output voltage and current were measured with the digitizers. Using a formula created
in the 14585A software, the efficiency, shown in the pink trace, is displayed as a percentage
directly on the screen. The results range from 80% at full load to about 93% at the
maximum efficiency point. To watch a video of this test in action, go to AgilentTube
and look for: DC to DC Converter Efficiency Measurements. Moving on to an example of turn-on
time and rise time testing, here you see the results of an actual turn-on test. Marker #1
is placed at the beginning and Marker #2 is placed at the point where the output voltage
is at 90%. The time difference between the markers is 1.5 milli-seconds. Clearly,
the high-speed digitizer in the N6782A is essential for this test. To watch a video
of this test in action, go to AgilentTube and look for: Testing a DC to DC Converter
As you have seen here, thorough testing of this key component used in mobile devices
involves capability beyond that of conventional test equipment. The Agilent N6705B with two
N6782A SMU’s uniquely provides the flexible source, dynamic load, and high-speed measurements
you need for accurate design verification of DC to DC converters. In addition, the 14585A
Control and Analysis software automates this operation and dramatically improves your design
effort efficiency. For more info, visit the Agilent website at www.agilent.com/find/N6782