Texas Instruments | Precision current measurements on high-voltage power-supply rails (Rev. C) | Application notes | Texas Instruments Precision current measurements on high-voltage power-supply rails (Rev. C) Application notes

Texas Instruments Precision current measurements on high-voltage power-supply rails (Rev. C) Application notes
Precision current measurements on high-voltage powersupply rails
Scott Hill, Current Sensing Products
Current is a signal that can provide valuable insight
into how a system is operating. Under defined
conditions, the amount of current required to perform a
task is consistent, making the current information a
useful indicator to determine if the system is operating
within expectations. There are multiple measurement
methods and locations where current is measured to
evaluate this informative signal.
Low-Side Sensing
One current measurement location is in the return path
to ground of a particular load or system. The device
requirements for this location are minimal, requiring
only an amplifier capable of handling a common-mode
signal reaching down to ground. As shown in Figure 1,
placing a small current sensing resistor (also called
shunt resistor) in series with the system return path to
ground develops a voltage across the resistor
proportional to the current.
BATTERY
VOUT = (ILOAD x RSHUNT) x (1+RF/RI)
VOUT = VSHUNT x Gain
LOAD
+
-
ILOAD
Table 1. Amplifiers for Current Sensing
VCM
=0V
VCM
>0V
Benefit
Disadvantage
Operational
amplifier
+
x
Low Cost
Accuracy,
Low-Side
Difference
amplifier
+
+
High-Side
Low Gain,
Cost
Instrumentation
amplifier
+
x
Accuracy,
High Gain
Low-Side,
Cost
Current sense
amplifier
+
+
High-Side,
High Gain,
Accuracy
—
One of the drawbacks to low-side sensing is the loss
of the direct connection to the system ground for the
load being monitored. As shown in Figure 2, when
current passes through the shunt resistor, the voltage
developed across the component changes, causing
the system reference to deviate from the ground
potential of the monitored load. This varying reference
connection can be problematic if the system is not able
to accommodate the ground potential moving up and
down proportionally to the system current.
VOUT
BATTERY
Full-Scale
RSHUNT
Current
LOAD
0A
Figure 1. Low-Side Current Sensing
VSHUNT
RF
RSHUNT
RI
+
Many types of amplifiers are able to accommodate this
low-side capability with an input voltage range down to
ground. Standard operational amplifiers, difference
amplifiers, instrumentation amplifiers, and current
sensing amplifiers are all capable of common-mode
input ranges inclusive of ground.
Table 1 provides an overview on how each of these
four amplifier types compare for current-sensing
applications.
SBOA165C – July 2016 – Revised February 2019
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VSHUNT
(System Reference)
Full-Scale
0V
Time (s)
Figure 2. Varying Load Reference
In addition to the varying system ground, some fault
conditions can be difficult to detect with a low-side
measurement location. If a short-circuit condition
results in current flowing through another path to
ground other than through the shunt resistor, the event
is not detectable by the low-side amplifier.
Precision current measurements on high-voltage power-supply rails Scott Hill,
Copyright © 2016–2019, Texas Instruments Incorporated
Current Sensing Products
1
www.ti.com
High-Side Sensing
VCM upto 40V
A challenge associated with the high-side
measurement location is that the amplifier must
interface with large input voltage rails, such as highvoltage batteries. A typical signal chain path for
measuring current is to amplify the voltage developed
across a current sensing resistor, and direct that
amplified signal to an analog-to-digital converter
(ADC). The input range of an ADC (whether discrete
or integrated within a microcontroller) is relatively small
compared to the voltage rails being monitored in
communication and industrial equipment. Commonmode voltage requirements can exceed 60 V, requiring
an amplifier capable of input signals far exceeding the
allowable input range for the low-voltage component.
Current sense amplifiers are dedicated amplifiers
developed specifically to accommodate these highvoltage input levels, while keeping the lower voltage
components following the amplifier within the linear
input range and protecting them from overvoltage
conditions.
The INA190 current sense amplifier accommodates
the requirement of monitoring high-voltage power rails
and interfacing with lower voltage components, while
being powered by a supply voltage as low as 1.7 V, as
shown in Figure 3.
In the event the system is placed into a shutdown or
sleep state, low-voltage supplies powering the ADCs,
microcontrollers, and signal path amplifiers can
potentially be turned off. However, batteries remain
connected to the measurement circuitry even if the
monitoring amplifiers are powered down. The INA190
features a capacitively coupled input stage that
eliminates unnecessary current draw from the battery
when disabled, or when the power supply is zero.
In always-on scenarios, the current sense amplifier
input circuitry is specifically designed to accommodate
the entire input range independent of the device
supply voltage. The INA190 can withstand the full 40-V
input voltage at the input pins, regardless of whether a
supply voltage is present or not, while drawing very
little current from the supply, and without being
damaged.
2
IN A190
Capacitive ly
Couple d
Amplifier
+
OUT
-
LOAD
Figure 3. INA190: Dedicated Current-Sensing
Amplifier
Alternate Device Recommendations
For applications that need the performance of the
INA190 in a smaller package with lower voltage enable
thresholds, the INA191 is offered in a small 0.96-mm2
chip-scale package. For applications with lower
performance requirements, using the INA199 still takes
advantage of the benefits of the dedicated current
sense amplifier. For higher voltage requirements, the
INA240 provides an input common-mode voltage
range reaching up to 80 V, and features enhanced
PWM rejection circuitry for applications with large input
voltage transitions, such as motor control and
switching power supplies. The INA301 current sense
amplifier features an onboard comparator to perform
overcurrent detection on chip.
Table 2. Alternate Device Recommendations
Device
Optimized Parameter
Performance Trade-Off
INA199
Lower Cost
IQ, IB, VOS, Gain Error
INA191
Small Size, 1.8 V Enable,
Accuracy
Package: WCSP-6
INA240
High VCM: -4 V to +80 V,
Bandwidth
IQ, IB, Package: TSSOP-8
INA301
Signal Bandwidth,
Onboard Comparator
IQ, IB, Package: MSOP-8
Table 3. Related TI Tech Notes
SBOA160
High Precision, Low-Drift In-Line Motor Current
Measurements
SBOA162
Measuring Current To Detect Out-of-Range
Conditions
SBOA163
High-Side Motor Current Monitoring for OverCurrent Protection
SBOA167
Integrating The Current Sensing Signal Path
Precision current measurements on high-voltage power-supply rails Scott Hill,
Current Sensing Products
VVS = 1.7V t o 5.5V
ENABLE
RSHU NT
Measuring current at the high-side of the load, or
directly in series with the power rail being monitored
and the remainder of the circuit, avoids both the
varying system reference and alternate short-circuit
path issues of low-side current measurements. The
high-side location allows for measuring the entire
system current, so any excess current through
unintended paths is detected. Moving away from the
low-side location eliminates the varying system ground
due to the current induced shunt voltage.
SBOA165C – July 2016 – Revised February 2019
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