What are the precautions for using current monitors?

As we can now see from the equations for newer high-side monitors, the impact of external resistors on CMRR is no longer an issue because MRR (typically greater than 90dB) is now dominated by the integrated amplifier. Integrating the current sensing function into a single IC has the following advantages:

Tight tolerances for active and passive integrated devices

Figure 1. Simplified schematic of a bidirectional high-side current monitor (MAX9928/MAX9929) with SIGN output indicating current direction.

Figure 2. Unidirectional high-side current monitor (MAX4372)

Figure 3. Another unidirectional high-side current monitor (MAX4172)

Figure 4. Alternative architecture for unidirectional high-side current monitor (MAX4173)

Things to consider when choosing RSENSE

When designing any type of current monitor, careful selection of the current-sense resistor (RSENSE) is very important and essential. RSENSE should be selected according to the following criteria:

Voltage losses: High RSENSE values ​​cause the supply voltage to decrease through IR losses. The lowest RSENSE value achieves the lowest voltage loss.

Accuracy: High RSENSE values ​​enable low current measurements with higher accuracy because the voltage offset and input bias current offset are much smaller than the sense voltage.

Efficiency and Power Dissipation: At high currents, I²R losses in RSENSE are relatively large, so this should be considered when selecting resistor values ​​and power dissipation ratings (watts). Excessive temperature of the detection resistor also causes its resistance value to drift.

Inductance: If ISENSE has a large high-frequency component, RSENSE must have low inductance. Wirewound chip resistors have the highest inductance, and metal film resistors are slightly better, but low-inductance metal film resistors (available below 1.5Ω) are recommended. Unlike metal film and wirewound chip resistors (that is, spirally wound around a core), low-inductance metal film resistors are composed of straight metal strips.

Cost: For RSENSE cost-critical applications, the PCB trace can be used as the sense resistor (Figure 10). Since the accuracy of copper resistors is not high, a potentiometer is needed to adjust the full-scale current. For systems with wide temperature variations, the temperature coefficient of copper resistance is quite high (approximately 0.4%/°C).

Figure 5. This high-side current monitor (MAX4172) uses PCB traces as RSENSE

High Side Monitor Applications

The circuit shown in Figure 6 is a variable linear current source. IC1 converts the R1 current into a proportional output voltage, causing the voltage regulator (IC2) to produce a regulated output current. To set IOUT to a regulated current between 0mA and 500mA, apply a voltage from 5V to 0V on ICONTROL (5V sets IOUT = 0mA, 0V sets IOUT = 500mA). As an alternative, you can add a D/A converter as shown to digitally control IOUT. For 12-bit resolution (60µA/LSB), the DAC can be a parallel-input MAX530 or serial-input MAX531; for 10-bit resolution (250µA/LSB), the DAC can be a parallel-input MAX503 or series-input MAX504.

Figure 6. Variable linear current source (MAX603)

The circuit shown in Figure 7 is a 0–5A programmable current source that generates 0A to 5A current from a 4V to 28V supply. It has two advantages: a 12-bit D/A converter allows it to be digitally programmed; and a switch-mode step-down regulation converter (IC1) making it more efficient than alternative current sources using linear transistors. Applications include overcurrent protection, 4–20mA systems, battery chargers, high-brightness LED control, GSM base station power supplies, and H-bridge motor control.

Figure 7. 0–5A Programmable Current Source (MAX4173)

The widespread application of the Universal Serial Bus (USB) has led to the development of various 2.7V to 5.5V power supply overcurrent protection circuits, but there are few products above this voltage range. The shunt shown in Figure 7 operates from a 26V supply voltage and uses a programmable current threshold for triggering.