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TECHNICAL ARTICLE
SMU1
Source V
Measure I
Figure 1
Line-regulation test
A line-regulation test measures a regulator’s ability to
maintain the specified output voltage under a constant
load as the input voltage is varied. During this test, the
output of SMU1 sweeps from a low voltage to a high
voltage, while SMU2 sinks a constant current. Typically,
the output voltage should vary less than 100mV.
Load-regulation test
A load-regulation test measures a regulator’s ability to
maintain the specified output voltage under varying load
current, while the input voltage remains constant. The
LDO regulator’s output voltage should vary less than
100mV.
For this test, SMU1 outputs a fixed input voltage.
SMU2 once again sources a fixed voltage less than the
regulator’s expected output voltage so that it will sink
current. By setting SMU2’s current limit to different
levels, you can simulate different load magnitudes and
measure how well the regulator operates under varying
loads.
Dropout voltage test
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Voltage
Regulator
GND
To measure a regulator’s dropout-voltage, SMU1 sweeps
from a maximum input voltage to a voltage below the
regulator’s expected drop out voltage, while SMU2 acts
as a load and sinks the output current. To configure
SMU2 to act as a load, program SMU2 to source a voltage
lower than the expected output voltage of the regulator.
Set the current limit to create the desired constant load
condition. Measuring both the input and output voltages
will allow you to determine easily the dropout voltage,
which is the input voltage value at which the output starts
to fall below its specified, regulated output level.
Quiescent current
Quiescent current is the difference between a regulator’s
input current and its output current. To measure
quiescent current, set SMU1 to sweep the regulator’s
input voltage range. Once again, configure SMU2 to
sink by programming its voltage to a value lower than
the output voltage of the regulator. While sweeping
the input voltage, measure both the input current and
output current. The difference in the two measurements
is the regulator’s quiescent current. By varying the
programmed current limit of SMU2, you can determine
how the quiescent current varies as a function of the
output current.
The SMU instrument’s versatility makes these types of
measurements a snap. Also keep in mind that you can
make these measurements manually, as you might do in
a development lab, or automatically, as you might do in
production.
About the Author
SMU2
Source V
Measure I
Robert Green is a Senior Market Development
Manager at Keithley Instruments focusing on low level
measurement applications. During his 20-year career at
Keithley, Mr. Green has been involved in the definition
and introduction of a wide range of products including
picoammeters, electrometers, digital multimeters, and
temperature measurement products. He received a
B.S. in Electrical Engineering from Cornell University
and an M. S. in Electrical Engineering from Washington
University, St. Louis, Missouri. ■
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