Current monitoring requires bidirectional current detection. How to achieve the most efficiency?
Latest update time：2024-04-10
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A growing number of applications require fast and accurate current monitoring, including autonomous vehicles, factory automation and robotics, communications, server power management, Class D audio amplifiers and medical systems.
In many of these applications, bidirectional current sensing is required and this needs to be accomplished efficiently and with minimal cost.
Although it is possible to build a bidirectional current-sense amplifier (CSA) using a pair of unidirectional current-sense amplifiers (CSA), the construction process can be complex and time-consuming. This process would involve either independent rail-to-rail op amps combining the two outputs into a single-ended output, or using two analog-to-digital converter (ADC) inputs on a microcontroller, which would require additional microcontroller coding and machine cycles. Finally, using two unidirectional CSAs to build a bidirectional CSA - plus the additional parts required to consolidate it into a bidirectional solution, will consume more board space, and more devices will reduce reliability and increase Large inventory requirements. Ultimately, both cost and design schedule may exceed expectations.
To avoid the above situation, designers can use integrated, high-speed, accurate bidirectional CSA. Designers can choose an integrated bidirectional CSA with an internal low-inductance shunt resistor for the most compact solution, or a CSA using an external shunt for more flexibility in design and layout.
This article reviews the implementation requirements for two-way CSA and the advantages of a more integrated approach. Then, devices from
STMicroelectronics
,
Texas Instruments
, and
Analog Devices
are used as examples to introduce in detail, including the key parameters of each device and their different characteristics. Finally, this article shows how to get started designing with these devices, including associated reference designs/evaluation kits/development kits, as well as design and implementation tips.
How to use two one-way CSA
Bidirectional CSA circuits can be constructed in a variety of ways using two unidirectional CSAs (Figure 1). The Analog Devices MAX4172ESA+T used in the example on the left does not have internal load resistors, so discrete components R a and R b are used . In the example on the right, the MAX4173TEUT+T includes a 12 kΩ load resistor to convert its current output to a voltage.
Figure 1: A bidirectional current-sensing application consisting of two unidirectional current-sense amplifiers can be implemented using either an external load resistor or an internal load resistor (right). (Image source: Analog Devices)
Although two load resistors are not required, a 1 nF capacitor is added to the feedback of the MAX4173TEUT+T circuit to stabilize the Part B control loop. In both cases, the output currents from the two CSAs are combined using
the MAX4230AXK+T
general-purpose op amp.
The part count for both methods is higher than when using a single bidirectional CSA. In addition to the higher parts count,
the PC board layout is more complicated because the two unidirectional CSAs need to be placed close to the VSENSE resistor.
Bidirectional CSA is a multifunctional device with a wide range of applications. For example, in a three-phase servo motor system, two CSAs can be used to determine the instantaneous winding currents of all three phases without any further calculations or anything related to pulse width modulation (PWM) pulse phase or duty cycle. information (Figure 2).
Figure 2: In a three-phase servo motor application, two bidirectional CSAs can be connected to the sense resistors of phase 1 (RSENSEΦ1) and phase 2 (RSENSEΦ2) to produce a voltage that represents the current in the third phase winding. (Image source: Analog Devices)
According to Kirchhoff's law, the sum of the currents in the first two windings is equal to the current in the third winding. This circuit uses two
MAX40056TAUA+
bidirectional CSAs to measure the two-phase current and
a MAX44290ANT+T
general-purpose op amp for current summation. Since all three amplifiers have the same reference voltage, the ratio measurement method results.
In another example, a Class D audio amplifier, a bidirectional CSA such as Texas Instruments'
INA253A1IPW
, can be used to accurately measure the speaker's load current (Figure 3).
Figure 3: In Class D audio designs, a bidirectional CSA (INA253) can be used for speaker enhancement and diagnostics. (Image source: Texas Instruments)
Real-time measurements of speaker load current can be used to diagnose and optimize amplifier performance by quantifying key speaker parameters and changes in these parameters, including:
-
Coil resistance
-
Speaker impedance
-
Resonant frequency and peak impedance at resonant frequency
-
Real-time ambient temperature of the speaker
Parasitic resistance and inductance are issues of concern when implementing current sensing circuits. In addition, excessive soldering and parasitic resistance can also cause detection errors. Typically a four-terminal current sense resistor is used. If a four-terminal resistor is not an option, the Kelvin PC board layout technique should be used (Figure 4).
Figure
4
: The Kelvin sense trace should be as close as possible to the solder contact pad on the current sense resistor. (Image source: Analog Devices)
将开尔文检测迹线置于尽可能靠近电流检测电阻的焊接接触点的位置会最大限度地减少寄生电阻。开尔文检测迹线的间距越大，就越会出现由于额外迹线电阻造成的测量误差。
正确选择检测电阻是将寄生电感降至最小的一个重要方面。由于电压误差与负载电流成正比，因此应尽量减少封装电感。一般来说，绕线电阻的电感值最大，标准金属膜器件的电感值处于中等水平。对于电流检测应用，一般推荐使用低电感金属膜电阻。
分流电阻的值是在动态范围和功率耗散之间进行权衡的结果。对于大电流检测，建议使用低值分流器，以尽量减少热耗散 (I
²
R)。在低电流检测中，使用较大的电阻值能够将失调电压对检测电路的影响降至最低。
大多数 CSA 依靠外部分流器测量电流，但也有一些 CSA 使用内部分流器。虽然使用内部分流器的设计外形更紧凑，器件更少，但需要进行一些权衡，具体包括：由于分流器值预先确定，所以灵活性较差；相比外部分流器，需要更大的静态电流；可测量的电流大小会受到内部分流器容量的限制。
使用 STMicroelectronics 的
TSC2011IST
，设计人员可以充分发挥其精密特性，使用低电阻外部分流器，从而最大限度地减少功率耗散（图 5）。这款双向 CSA 旨在为数据采集、电机控制、螺线管控制、仪表、测试和测量以及过程控制等应用提供精确的电流测量。
图 5：TSC2011IST 包括一个关断引脚 (SHDN)，可以最大限度地节能，其工作温度为 -40 至 125℃。（图片来源：STMicroelectronics）
TSC2011IST的放大器增益为 60 V/V，集成了电磁干扰 (EMI) 滤波器，具有 2 (kV) 人体模型 (HBM) 静电放电 (ESD) 容限（根据 JEDEC JESD22-A114F 标准的规定）。TSC2011 可以检测到低至 10 mV 的满量程电压降，以实现稳定测量。凭借 750 kHz 增益带宽积和 7.0 V/
µ
s压摆率，该器件确保了高精确度和快速响应。
设计人员可以借助
STEVAL-AETKT1V2
评估板快速上手 TSC2011IST（图 6）。该器件可以在 -20 V 到 +70 V 的宽共模电压范围内检测电流。TSC2011IST 的特点：
-
增益误差：最大 0.3%
-
失调漂移：最大 5 µ V/°C
-
增益漂移：最大 10 ppm/°C
-
静态电流：关断模式下 20 µ A
图 6：STEVAL-AETKT1V2 评估板包括主板和一个包含 TSC2011IST 的子卡。（图片来源：STMicroelectronics）
Texas Instruments 的
INA253A1IPW
集成了一个 2 mΩ、0.1% 低电感分流器，支持高达 80 V 的共模电压（图 7）。INA253A1IPW为设计人员提供了可抑制大 dv/dt 信号的增强型 PWM 抑制电路，从而实现了针对如电机驱动、螺线管控制等应用的连续、实时的电流测量。内部放大器具有精密的零漂移拓扑结构，且共模抑制率 (CMRR) 大于 120 dB DC CMRR 和 90dB AC CMRR（50 kHz 时）。
图 7：典型应用中的 INA253A1IPW 双向 CSA。该器件具有内部分流器，可在 -40 至 +85°C 的温度范围内测量
±15 A 连续电流。（图片来源：Texas Instruments）
通过使用相关的
INA253EVM
评估板上的测试点访问 INA253A1IPW 的功能引脚，设计人员就可以加快基于该 CSA 的系统设计开发（图 8）。这种双层板的尺寸为 2.4 ×4.2 英寸，用 1 oz 的铜制作而成。
图 8：该双层 INA253EVM 器件的尺寸为 2.4 × 4.2 英寸，用 1 oz 的铜制作而成。该器件的底层没有任何元件，只是一个坚实的铜制地平面，用来为返回电流提供低阻抗路径。（图片来源：Texas Instruments）
该 pc 板上包括了最小的支持电路，并且可以根据需要重新配置、移除或旁路掉各种功能。INA253EVM具有以下功能：
-
三个 INA253A1IPW 器件
-
所有针脚都易于接触
-
在整个 -40 至 +85°C 的温度范围内，支持 ±15A 的电流通过 INA253 CSA 的电路板布局和结构
-
将支座放在 pc 板上，可用于除默认配置外的其他配置
该器件的底层没有任何元件，只是一个坚实的铜制地平面，用来为返回电流提供低阻抗路径。
Figure 9: The LT1999IMS8-20#TRPBF is a bidirectional CSA in full-bridge armature current monitoring applications. (Image source: Analog Devices)
The LT1999IMS8-20#TRPBF is AEC-Q100 qualified for automotive applications and includes a shutdown mode to minimize power consumption. This device uses an external shunt to measure the direction and magnitude of current flow. The device generates an output voltage proportional to a reference midpoint between voltage and ground. Designers can choose to use an external voltage to set the reference level.
When VSHDN (Pin 8) is driven to 0.5 V ground, the LT1999IMS8-20#TRPBF enters a low-power shutdown state consuming approximately 3 μA. If the input pins (+IN and -IN) are biased within 0 to 80 V (no differential voltage applied), their current consumption is approximately 1 nA. An internal first-
order differential low-pass EMI suppression filter reduces EMI susceptibility and helps eliminate high-frequency signals that exceed the device bandwidth.
To facilitate testing of the LT1999 series, Analog Devices provides
the
1698A
demonstration board. The board amplifies the voltage drop across the on-board current-sense resistor and produces a bidirectional output voltage proportional to the current flowing through the resistor. Designers can choose from three fixed gains: 10 V/V (DC1698A-A), 20 V/V (DC1698A-B), and 50 V/V (DC1698A-C).
To improve rejection of common-mode input PWM edges in designs that control inductive loads such as solenoids and motors, designers can use the MAX40056TAUA+ (Figure 10). As mentioned above in Figure 2, the MAX40056TAUA+ is a bidirectional CSA that can handle
slew rates of ±500 V/
µ
s and above.
The device has a typical CMRR of 60dB (50 V, ±500 V/
µs
input) and 140 dB DC. It has a common-mode range from -0.1 V to +65 V and provides protection against inductor bounce voltages as low as -5 V.
Figure 10: The MAX40056TAUA+ includes an internal 1.5 V voltage reference, enhanced PWM rejection, and an internal integrated window comparator to detect positive and negative overcurrent conditions (lower left, driven by the CIP input). (Image source: Analog Devices)
The MAX40056TAUA+ device has an internal 1.5 V voltage reference. This reference voltage is used for a variety of purposes, including:
-
Driving Differential Analog-to-Digital Converters
-
Offset the output to show the detected direction of current flow
-
Direct current into an external load to mitigate performance degradation
When a higher full-scale output swing is useful or when the supply voltage is higher than 3.3 V, the designer can override the internal reference with a higher external voltage reference. Finally, designers can use an internal or external reference to set the threshold to trip the integrated overcurrent comparator, providing an immediate overcurrent fault signal.
The MAX40056EVKIT#
evaluation kit
for the MAX40056TAUA+
provides designers with a proven platform for developing high-precision, high-voltage, bidirectional CSA applications such as solenoid drivers and servo motor controllers.
in conclusion
Fast, accurate current monitoring is required in a wide variety of applications, ranging from autonomous vehicles, factory automation and robotics, to communications, server power management, Class D audio amplifiers and medical systems. In many situations, bidirectional current sensing is required.
Fortunately, designers can choose from a variety of integrated bidirectional CSAs and their associated development platforms to implement fast, accurate bidirectional current monitoring quickly and efficiently.
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