High-side current detection circuit and principle

High-side/low-side current-sense circuit
The current-sense resistor of the low-side current-sense circuit is connected in series to the ground (Figure 1), while the current-sense resistor of the high-side current-sense circuit is connected in series to the high-voltage terminal (Figure 2). Both methods have their own characteristics: the low-side current-sense method adds an extra resistor in the ground loop, and the high-side current-sense method has to deal with a larger common-mode signal.
The low-side current-sense op amp shown in Figure 1 uses the ground level as the reference level, and the current-sense resistor is connected to the non-inverting terminal. The common-mode signal range in the input signal of the op amp is: (GNDRSENSE*ILOAD). Although the low-side current-sense circuit is relatively simple, there are several fault conditions that the low-side current-sense circuit cannot detect, which will put the load in a dangerous situation. The use of high-side current-sense circuit can solve these problems. The
high-side current-sense circuit is directly connected to the power supply terminal, and can detect any faults in the subsequent circuit and take corresponding protection measures. It is particularly suitable for automatic control applications because the chassis is usually used as the reference ground in these application circuits.

Traditional high-side current-sense circuit
There are many implementation schemes for the traditional high-side/low-side current-sense method, most of which are based on discrete or semi-discrete component circuits. High-side current sensing circuits usually require a precision op amp and some precision resistors and capacitors. The most commonly used high-side current sensing circuit uses a differential op amp to gain and shift the signal level from the high side to the reference ground (Figure 3):
VO = IRS * RS; R1 = R2 = R3 = R4
This solution has been widely used in practical systems, but this circuit has three major disadvantages:
1) The input resistance is relatively low, equal to R1;
2) The input resistance at the input end generally has a large error value;
3) The resistor matching degree must be high to ensure acceptable CMRR. A 1% change in any resistor will reduce the CMRR to 46dB; a 0.1% change will increase the CMRR to 66dB, and a 0.01% change will increase the CMRR to 86dB. High-side current sensing requires high measurement skills, which has promoted the development of high-side current sensing integrated circuits. However, low-side current sensing technology does not seem to have made corresponding progress.

Using integrated differential op amps to implement high-side current sensing
The circuit using differential op amps for high-side current sensing is more convenient to use because many integrated circuit solutions have been introduced recently. The integrated circuit includes a precision op amp and well-matched resistors, with a CMRR of up to about 105dB. The MAX4198/99 is such a product, with a CMRR of 110dB, a gain error better than 0.01%, and a small 8-pin mMAX package.
The dedicated high-end current detection circuit contains all the functional units to complete high-end current detection. It can detect the high-end current at a common-mode voltage of up to 32V and provide a current output proportional to it with the ground level as the reference point. Applications that require accurate measurement and control of current, such as power management and battery charging control, are suitable for this solution.
The current sensing resistor used in MAXIM's high-end current sensing op amp is placed between the high-end of the power supply and the power input of the circuit being detected. The current sensing resistor is placed on the high side without adding additional impedance to the ground loop. This technology improves the performance of the entire circuit and simplifies the layout requirements.
MAXIM has launched a series of bidirectional or unidirectional current detection ICs. Some bidirectional current detection ICs have built-in current sensing resistors that can detect the current flowing into or out of the circuit being detected and display the current direction through a polarity indicator pin. Gain-adjustable current detection ICs, fixed-gain (+20V/V, +50V/V, or +100V/V) current detection chips, or fixed-gain current detection ICs including single and dual comparators all use small-volume packages, such as SOT23, to meet applications with demanding size requirements. Figure 4 shows a high-end current detection circuit using MAX4173.

The relationship between the output voltage and the current-sense resistor in the figure is:
Vo=RGD*(Iload*Rsense)/RG1)*b, where b is the mirror current coefficient.
The above formula can be further simplified to: Vo=Gain*Rsense*Iload;Gain= b*RGD/RG1.
Gain is 20 (MAX4173T), 50 (MAX4173F), 100 (MAX4173H). [page]

It can be seen from the above calculation formula that CMRR is determined by the process of the internal integrated current-sense circuit (typical value>90dB) and is no longer affected by external resistance.
The use of integrated current-sense circuits has the following advantages:
1. Good consistency of devices
2. Excellent temperature drift characteristics
3. Small size
4. Low power consumption
5. Easy to use
Precautions for selecting current-sense resistors
The current-sense resistor RSENSE should be selected according to the following principles:
1. Voltage loss: A current-sense resistor with too large a resistance value will cause the power supply voltage to decrease by the value of IR. In order to reduce voltage loss, a current-sense resistor with a small resistance value should be selected.
2. Accuracy: A larger current-sense resistor can achieve higher measurement accuracy for small currents. This is because the larger the voltage on the current-sense resistor, the smaller the impact of the offset voltage and input bias current of the op amp.
3. Efficiency and power consumption: When the current is large, the power consumption I2R on RSENSE cannot be ignored. This needs to be noted when considering the current-sense resistor and power consumption. If the current-sense resistor is allowed to heat up, the resistance value can be larger.
4. Inductance: If ISENSE contains a lot of high-frequency components, the inductance of the current-sense resistor should be very small. Wirewound resistors have the largest inductance, and metal film resistors are better.
5. Cost: If the price of a suitable current-sense resistor is too high, an alternative solution can be used (Figure 5). It uses the printed wire of the circuit board as the current-sense resistor. Because the "resistance" of the copper wire on the printed circuit board is not accurate, a potentiometer is required in the circuit to adjust the full-scale current value. In addition, the temperature drift of the copper wire is large (about 0.4%/℃), which needs to be considered in systems operating over a wide temperature range.

Adjustable linear current source
The adjustable linear current source (Figure 6) is a typical application circuit composed of a high-end current detector. IC1 converts the current of R1 into a corresponding proportional voltage signal, controls the voltage regulator chip IC2 to produce a stable output current, and the D/A converter can provide digital control of IOUT. To achieve 12 BIT accuracy (60mA per LSB), use the MAX530 with a parallel interface or the MAX531 with a serial interface. For 10 BIT accuracy (250mA per LSB), use the MAX503 with a parallel interface or the MAX504 with a serial interface.