High end/low end current detection circuit

The detection resistor of the low-end current detection circuit is connected in series to ground (Figure 1), while the detection resistor of the high-end current detection circuit is connected in series to the high voltage terminal (Figure 2). Both methods have their own characteristics: the low-end current detection method adds additional winding resistance in the ground circuit, while the high-end current detection method needs to handle larger common mode signals. For more detailed information about wire wound resistors, please click here: http://www , dzsc, com/product/searchfile/898, HTML

The detection resistor of the low-end current detection circuit is connected in series to ground

The current checking resistor of a high-end current checking circuit is connected in series to the high voltage terminal

The low-end current checking operational amplifier shown in Figure 1 uses the ground level as the reference level, and the current checking resistor is connected to the positive phase terminal. The common mode signal range in the input signal of the operational amplifier is (GNDRSENSE * ILOAD). Although low-end current detection circuits are relatively simple, there are several fault states that the low-end current detection circuit cannot detect, which can put the load in a dangerous situation. Using high-end current detection circuits can solve these problems.

The high-end current detection circuit is directly connected to the power supply end, which can detect any faults in the subsequent circuit and take corresponding protective measures. It is particularly suitable for automatic control applications, as the casing is usually used as a reference ground in these application circuits.

Traditional high-end current detection circuit

There are multiple implementation schemes for traditional high-end/low-end current detection methods, with the vast majority based on discrete or semi discrete component circuits. High end current detection circuits typically require the use of a precision operational amplifier and some precision resistors and capacitors. The most commonly used high-end current detection circuit uses a differential operational amplifier for gain amplification and shifts the signal level from the high end to the reference ground (Figure 3):

Using differential operational amplifiers for gain amplification and shifting the signal level from high end to reference ground

VO=IRS * RS; R1=R2=R3=R4

This scheme has been widely applied in practical systems, but the circuit has three main drawbacks:

1) The input resistance is relatively low, equal to R1;

2) The input resistance at the input end generally has a significant error value;

3) 129740-002 requires a high matching degree of resistance to ensure an acceptable CMRR. Any 1% change in resistance will reduce the CMRR to 46dB; A 0.1% change brings CMRR to 66dB, and a 0.01% change brings CMRR to 86dB. High end current detection requires high measurement skills, which promotes the development of high-end current detection integrated circuits. However, there seems to be no corresponding progress in low-end current detection technology.

Implementing high-end current detection using integrated differential operational amplifiers

The use of differential operational amplifiers for high-end current detection circuits is more convenient to use, as many integrated circuit solutions have recently been introduced. The integrated circuit includes a precision operational amplifier and a well matched resistor, with a CMRR of around 105dB. MAX4198/99 is such a product, with a CMRR of 110dB, a gain error of better than 0.01%, and a small 8-pin mMAX package.

The dedicated high-end current detection circuit contains all functional units to complete high-end current detection, which can detect high-end current at a common mode voltage of up to 32V and provide proportional current output based on ground level as a reference point. Applications that require precise measurement and control of current, such as power management and battery charging control, are suitable for this approach.

The current checking resistor used in the high-end current checking operational amplifier of 129740-002 is placed between the high end of the power supply and the input end of the tested circuit. The current checking resistor is placed at the high end without adding additional impedance to the ground circuit. 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 detection resistors, which can detect the magnitude of the current flowing into or out of the tested circuit and display the direction of the current through a polarity indicator pin. Current detection ICs with adjustable gain, fixed gain (+20V/V,+50V/V, or+100V/V) current detection chips, or fixed gain current detection ICs including single and double comparators, all use small volume packaging, such as SOT23, which can meet demanding size requirements for applications. Figure 4 shows a high-end current detection circuit composed of MAX4173.