Texas Instruments | Automotive Current Monitoring Using High Speed Amplifiers | Application notes | Texas Instruments Automotive Current Monitoring Using High Speed Amplifiers Application notes

Texas Instruments Automotive Current Monitoring Using High Speed Amplifiers Application notes
Automotive Current Monitoring Using High Speed
Amplifiers
With vehicle electrification increasing and as fully
electric vehicles become more mainstream, the
number of electric motors and digital power control
systems in automobiles are expanding. Many of these
systems require high-speed current monitoring circuits
to ensure proper operation and to protect against
potentially damaging overcurrent conditions. One of
the most effective ways to accomplish this function is
to employ a low side current shunt monitoring circuit
like the one shown in Figure 1.
amplifier also has an operating temperature range of
-40 to 125 degrees Celsius and is AEC Q100 grade 1
qualified for automotive applications. The key
specifications for the OPA365-Q1 op amp are
summarized in Table 1.
Table 1. OPA365-Q1 Key Specifications
OPA365-Q1 Key Specifications
Inverter
GateDriver
Driver
Gate
Gate
Driver
Buffer
C2000Œ
Microcontroller
Comparator
Op Amp
SN74LVC2G74-Q1
D Flip Flop
LMV393-Q1
Comparator
TLV314-Q1
Op Amp
Figure 1. Automotive Current Monitoring Diagram
This circuit provides overcurrent and short-circuit
protection with the use of two op amps, two
comparators and a D-type flip-flop. The OPA365-Q1
operational amplifier (op amp) used in the circuit
performs a differential voltage measurement across a
shunt resistor. The voltage across the resistor
corresponds to the current that flows though the
inverter. It is important that the op amp used in this
part of the circuit has low noise and low offset to
minimize degradation of the low voltage signal
measured across the shunt resistor. It is also critical
that the amplifier has fast settling time and a high slew
rate in order to achieve a fast response time to detect
overcurrent or short-circuit conditions. High bandwidth
also enables a single high-gain stage to be
implemented in a single amplifier to drive the input of
the comparator and analog to digital converter (ADC)
in the circuit.
The zero-crossover, rail-to-rail, OPA365-Q1 CMOS op
amp is ideal for this application. It has 50MHz of
bandwidth, noise of only 4.5nV per root hertz and a
low offset of only 100 micro volts. The device also
features a fast settling time of only 0.3 micro seconds
and a slew rate of 25 volts per micro second. The
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50 MHz
Noise
4.5 nV/√Hz @ 100 kHz
Offset
100 μV
Settling Time
0.3 μs to 0.01%
Slew Rate
25 V/μs
Ambient Temperature
-40°C to 125°C
Package Size (Body)
2.9 mm x 1.6 mm SOT-23(5)
LMV393-Q1
OPA365-Q1
VCC / 2
Bandwidth
The TLV314-Q1 is used in the circuit as a voltage
follower and functions as a source for the reference
voltage. This amplifier provides high stability at unity
gain and has the ability to drive capacitive loads up to
300 pF. In this circuit, the TLV314-Q1 provides a
reference voltage of Vcc divided by 2 to the OPA365Q1. With this configuration, the output voltage range of
the OPA365-Q1 varies from 1/2 of Vcc to Vcc for one
direction of shunt current and from 1/2 of Vcc to GND
for the other direction of shunt current. This circuit is
shown below in Figure 2.
47 pF
100 k
3.3 V
VCC
10 k
Shunt_L
±
From Shunt
OPA365-Q1
To Comparators &
C2000Π0&8 $'&
10 k
Shunt_H
3.3 V
VCC
10 k
+
3.3 V
VCC
100 k
±
TLV314-Q1
+
10 k
Figure 2. Reference Voltage Circuit
Automotive Current Monitoring Using High Speed Amplifiers
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1
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The output of the OPA365-Q1 is connected to the
comparator circuit and the analog to digital converter
input on the C2000™ microcontroller. The C2000
TMS320F28052 microcontroller is part of TI’s
Piccolo™ family of MCUs. It has a 12-bit, 3.75 mega
sample per second ADC that is driven by the OPA365Q1 amplifier and can be used to monitor or log the
shunt current. The C2000 microcontroller has been
specifically designed for use in automotive motor
control and digital power applications.
Since a short circuit or overcurrent condition can
potentially be harmful, a comparator-based shut-off
circuit with an extremely fast response time is also
implemented. This comparator-based circuit can
disable the gate drivers in the system much faster than
the MCU. In the circuit, the OPA365-Q1 differential op
amp output is connected to the inputs of the dual
channel LMV393-Q1 comparator. This device
integrates two comparators into a single package and
provides a cost-effective, space-saving design for this
portion of the circuit. The comparators have trigger
levels that correspond to an overcurrent condition.
One comparator is set up in an inverted configuration
to make it possible to detect current peaks in the shunt
resistor in the opposite direction. When a overcurrent
or short-circuit current occurs on one of the phases of
the inverter, the responsible comparator detects it and
changes its output state accordingly. The dual
comparator circuit is shown in Figure 3.
When the comparator changes state, it triggers the
SN74LVC2G74-Q1 D flip-flop in the circuit to disable
the buffer between the MCU and gate drivers. When
the buffer is disabled, the gate driver input signals are
pulled down through the low-side resistors connected
to each control line and the gate drivers force the
Insulated-Gate Bipolar Transistors or IGBTs in the
system to turn off. This comparator based circuit has a
very fast response time that can potentially help
prevent damage from occurring to the system in the
event of a short circuit fault.
This current monitoring circuit is featured in a TI
Design, or reference design, for a high-voltage, highpower motor drive circuit for automotive heating,
ventilation and air conditioning (HVAC) compressors.
The reference design features a three-phase inverter
drive which is based on a discrete IGBT for driving
brushless DC (BLDC) motors for automotive A/C
compressors using sensorless torque control. More
information about the TI Design, including full design
schematics, can be downloaded from the following
web link: http://www.ti.com/tool/tida-01418. The circuit
board for the reference design is shown in Figure 4.
VCC
10 k
10 k
±
30 k
LMV393-Q1
To Flip Flop
2.4 k
Figure 4. TI Design TIDA-01418 Circuit Board
+
100 k
From OPA365-Q1
Amplifier
VCC
2.4 k
30 k
10 k
±
LMV393-Q1
To Flip Flop
With the number of electric motors and digital power
systems increasing in automobiles the need for highspeed current monitoring systems is becoming more
important. The zero-crossover, rail-to-rail, OPA365-Q1
op amp is in integral part in these systems. It has fast
settling time and a high slew rate in order to achieve a
fast response time to detect overcurrent or shortcircuits in these systems to prevent potentially
damaging conditions from occurring.
+
10 k
100 k
Figure 3. Dual Comparator Circuit
2
Automotive Current Monitoring Using High Speed Amplifiers
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