AN-1321: Common-Mode Transients in Current Sense Applications (Rev. 0)

AN-1321
APPLICATION NOTE
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Common-Mode Transients in Current Sense Applications
by Kristina Fortunado
INTRODUCTION
Current sense amplifiers are used in a variety of applications,
such as motor or solenoid control, load current monitoring, and
fault detection. In such applications, it is typical for the input
common-mode voltage to swing from ground to a certain highside supply. However, while a user may work with the assumption
that the input common-mode swings are limited to this highside supply, there are transient voltages that must be considered.
The result of these transients is that a supposed low voltage
application tends to appear as a high voltage application, and
the current sense amplifier must be robust enough to handle
these occurrences.
TRANSIENT VOLTAGES IN A MOTOR DRIVE
CIRCUIT
One can consider a motor drive circuit to gain insight into these
transient voltage events. The circuit shown in Figure 1 uses the
ADuM3223 to drive the gates of two MOSFETs in a half-bridge
configuration. The inputs of the ADuM3223 are driven with
inverted pulse-width modulation (PWM) signals with duty
cycles of 50%, enabling switching between the two MOSFETs.
VDD1
The node between the emitter of the high-side FET and the
collector of the low-side FET is the half-bridge point of the
motor drive circuit. This node becomes the connection to the
shunt resistor, RSH, and the motor load, represented by an
inductance, M. In this circuit, the AD8418, a current sense
amplifier, is used to monitor the differential voltage across the
shunt resistor. Since this differential voltage is typically a small
value in the range of millivolts, the common-mode voltage seen
by the current sense amplifier is essentially the voltage at the
half-bridge point, and is denoted as VCM in Figure 1.
When the low-side FET turns on, the half-bridge point is pulled
down to ground. When the low side FET switches off and the
high side FET turns on, the half-bridge point switches to the
bus voltage, VBUS. It is during this momentary switching that
transients become apparent. These transients are caused by the
fast switching speed of the load, along with the reactive nature it
presents to the driver.
VBUS
VDDA
VOA
VIA
GNDA
VCM
ADuM3223 VDDB
VIB
RSH
AD8418
VOUT
VOB
12606-001
M
GND1
GNDB
Figure 1. Motor Drive Circuit with the ADuM3223 and the AD8418
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Application Note
12606-003
12606-002
AN-1321
Figure 2. Common-Mode Voltage at the Half-Bridge Point
Figure 3. AD8418 Output at Rising Edge
Figure 2 shows the common-mode voltage taken at the halfbridge point with a switching frequency of 10 kHz and a bus
voltage of 15 V. A close look at the plot shows transients at both
swings of the common-mode voltage. The transient at the rising
edge reaches almost 8 V, which is more than 50% of the bus
voltage. At the falling edge, there is a transient of about −2.5 V.
For applications where higher bus voltages and faster switching
frequencies are used, the transients may effectively become
higher.
Figure 3 and Figure 4 show the typical response of the AD8418
to the common-mode voltage transients from the ADuM3223
motor drive circuit. The AD8418 output deviates from the
expected voltage by about 30 mV to 40 mV as the commonmode voltage switches, then settles back to the expected output
in a few microseconds. The capability of the current sense
amplifier to handle these high transients is dictated by its input
common-mode voltage range specification. Other amplifiers
may have an absolute maximum specification, shown either as
the common-mode voltage survival range or the continuous
input common-mode voltage.
The Analog Devices, Inc. line of current sense amplifiers are
designed to operate with a wide range of input common-mode
voltages. The AD8418, for example, has a common-mode
voltage survival range from −4 V to +85 V. For applications with
larger negative transients, current sense amplifiers such as the
AD8202 can survive common-mode voltages down to −8 V.
12606-004
CHOOSING AN AMPLIFIER
Figure 4. AD8418 Output at Falling Edge
CONCLUSION
Amplifier choice ultimately depends on the requirements of the
current sense application. It is important for the user to
recognize the occurrence of common-mode transient voltages
and to accommodate these when choosing the appropriate
amplifier. The varied input common-mode voltage ranges of the
line of current sense amplifiers from Analog Devices provide
the user with flexibility for these considerations.
©2014 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
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