Measuring the current in the system is a basic but powerful tool to monitor the state of the system. With the help of advanced technology, the physical size of electronic or electrical systems is greatly reduced, reducing power consumption and cost, without much compromise in performance. Each electronic device is monitoring its own health and status. These diagnostics provide important information required by the management system, and even determine its future design and upgrading.
There is an increasing need to measure various currents in the system, ranging from small current levels to a few amperes. For example, a high dynamic range that determines the current or consumption in the system can be seen in the following cases:
Sleep / inactive current, used to determine the overall load performance and battery / power estimation in addition to normal operation.
Ate/ test environment needs to deal with micro / low micro ampere current level to ampere current level, which requires R & D or production level test.
Production workshop environment to capture production problems (flux under IC, unnecessary welding short circuit or open circuit) and normal operation function test.
Industrial equipment monitoring, power consumption during startup and shutdown provides the health status of the equipment, for example, monitoring the normal current and leakage current in the equipment to determine its wear over time.
Figure 1 Current detection amplifier (CSA) + detection resistance.
Figure 1 shows a CSA with detection resistance. In higher voltage level (common mode level) applications up to 80V, a simple external current detection amplifier (CSA) (but with complex integrated circuit design, its architecture meets the accuracy and accuracy) and detection resistor are the solutions to most problems when measuring current. At present, the current detection amplifier has first-class accuracy and precision, which can meet the needs of realizing micro ampere current level, while still maintaining better signal-to-noise ratio (SNR) performance to provide the measurement resolution required by the system design.
However, it is not easy to choose an optimized CSA for designers. There are some trade-offs to consider (Figure 2):
Minimum detectable current (how low is the input offset voltage (V OS) converted to the device)
Maximum detectable current (converted to maximum input detection voltage (V sense))
Allowable power consumption on R sense
Figure 2 Design constraints to consider when using CSA and R sense.
Since the differential voltage range is set by the selection of the current detection amplifier, increasing the R sense value can improve the measurement accuracy of lower current values, but the power consumption is higher at higher current, which may be unacceptable. In addition, the range of the sensing current is reduced (I Min: I max).
Reducing the R sense value is more advantageous because it reduces the power consumption of the resistor and increases the sensing current range. Reducing the R sense value will reduce the SNR (SNR can be improved by averaging the noise at the input). It should be noted that in this case, the deviation of the equipment will affect the accuracy of the measurement. Generally, the calibration at room temperature is to improve the system accuracy, eliminate the offset voltage and increase the test cost of some systems.
In addition, the input differential voltage range (V sense) depends on the supply voltage or internal / external reference voltage and gain:
In any application that achieves a high current range, the goal is to maximize the dynamic range of the target accuracy budget, which is usually estimated by the following equation:
For most inputs, the offset voltage is about 10 μ CSA of V, V sense-range is usually 100mV. Note that if V sense_ If min is selected as 10xv OS factor, in the uncalibrated system, this can provide up to 3 octaves with an error of ± 10%. Similarly, if 100xv OS is selected, an error range of ± 1% can be achieved, but the dynamic range will be reduced to 2 tens of octaves. Therefore, there is a trade-off between dynamic range and accuracy: tightening the accuracy budget will reduce V sense_ Dynamic range specified by min and vice versa.
It should be noted that in the CSA + R sense system, R sense (tolerance and temperature coefficient) is usually the bottleneck of the overall accuracy of the system. This is still an effective way to monitor / measure system current in the industry because it is simple, reliable and affordable compared with other alternatives such as electricity meter, CSA of integrated chip resistor, discrete implementation of differential amplifier using operational amplifier and so on. Higher levels of tolerance and temperature coefficient detection resistors can be found, but at higher prices. The total error budget applied in the temperature range needs to be equal to the error generated by R sense.
Resistance free sensing solution:
The integrated current sensing device (U1) shown in Figure 3 below is a very useful and effective solution for applications that need to measure higher dynamic range currents from a few hundred microamps to a few amperes. The solution meets the following criteria:
Integrated sensing element (no resistance)
Current sensing dynamic range greater than 4 tens of octave
Current output function (0-1v out with 160 Ω load, compatible with all ADC / microcontroller inputs to achieve current).
Figure 3: 2.5V to 5.5V current sensing system with integrated current sensing elements
Instead of the external detection resistor, there is an integrated detection device between the V DD input and the load (LD) output, which can measure the system load current (I load) from 100ua to 3.3a. The internal gain module with a gain of 1/500 provides the output current of ISH, i.e. A 160 Ω resistor is connected from ish current output to GND and converted to V ish voltage output from 0V to 1V.
Under 3A load current, the voltage drop between Vdd and LD on the sensing element is about 60mV (Fig. 1), which is only equivalent to 180MW power consumption, while at a lower current value, 100 is detected μ The total error of range a is within 10% of this area (Figure 2). In addition, the power consumption is lower under higher current loads, and the improved error budget is still maintained at lower current levels. This scheme is superior to the traditional detection circuit of Fig. 1. Therefore, applications requiring a wider current detection range up to 3A detection can benefit from this scheme.
Resistance free sensing solution with extended line / input voltage:
Figure 4 is an extension of the input voltage range of Figure 3, in which the power supply voltage of U1 can now accept higher line voltage, up to 6V to 36V. The zener diode (D1) maintains the voltage between V DD and pFET (M1) gate at 5.6 v. Most of the high-voltage line is absorbed by M1, and the source of M1 is clamped to the input voltage of about 4v-4.5v DD from V, so as to keep the working voltage of U1 (V DD – V SS) within its normal working range (Fig. 3). Then, the source voltage of M1 offsets the gate voltage of M2 pFET. The M2 pFET source is located at V SS (U1) + V th (M2) to ensure that the U1 ish output is within the acceptable voltage range. Ish current output and R1 generate 0 to 1V output relative to GND.
Figure 4 6V to 36V current sensing system with integrated current sensing elements
The following is the experimental result of the circuit in Fig. 4.
Figure 1: relationship between voltage drop on internal sensing element and load current
Figure 2: gain error of ISH output and load current at different temperatures
Figure 3: functional relationship between max40016 power supply voltage (V DD – V SS) and V line
Figure 4 Load transient response of I load step change from 0 to 3a.
Figure 5 Power on transient response of 3A I load.
As shown in the figure, the resistance free sensing method makes it possible to design a 4-decade current sensing architecture with an extended operating range of up to 36V.
[Note: all data and charts are provided by Maxim integrated.]
The author, bich Pham, joined Maxim integrated in 2000 as a customer application engineer. He is now a senior member of the technical staff. He still focuses on helping customers solve real-world design challenges. Bich holds a bachelor’s degree in electrical engineering from San Jose State University in California.