Integrated Circuits for Current Measurement
Typically, integrated circuits (ICs) designed for measuring currents require external sense resistors with values in the range of a few tens of milliohms. Acquiring these specific resistors can be challenging, and the layout of the circuit board may adversely impact the measurement accuracy. National Semiconductor addresses this issue with the LM3822 and LM3824 ICs, available at www.national.com. These ICs come equipped with built-in sense resistors valued at only 3 mW, eliminating the need for external components. The measured value is converted into a digital format using a delta-sigma modulator, ensuring precise and convenient current measurements.
Digital Filtering and Pulse-Width Modulation
A digital filter processes the current’s average value every 50 ms (LM3822) or every 6 ms (LM3824). This filtered value is then compared to a digital ramp signal to generate a pulse-width modulated signal (PWM) proportional to the current level. According to the manufacturer, LM3822/24 stands out as the most accurate high-side current measurement IC available. The LM3824 offers an accuracy of ±2%. On the output side, LM3822/24 produces a PWM signal where the duty cycle (D) indicates the measured current value, along with its sign.
Interpreting Duty Cycle and Current Polarity
In the absence of current (zero current), the duty cycle is precisely 50%. Positive currents result in duty cycle values ranging from 50% to 95.5%, while negative currents result in duty cycle values ranging from 50% to 4.5%. For instance, a value of 95.5% corresponds to +1 A (or +2 A), and 4.5% corresponds to -1 A (or -2 A). Current is considered positive when it flows from SENSE to SENSE–.
IC Specifications and Voltage Operation
The LM3822/24 ICs operate within a supply voltage range of 2.0 V to 5.5 V and consume less than 150 µA of internal current. If the operation requires voltages above 5.5 V, a Zener diode shown in the schematic diagram with a 10-kΩ series resistor can be used. This Zener diode restricts the LM3822/24 operating voltage to a safe 4.7 V. However, note that in this scenario. The output signal is no longer referenced to ground but fluctuates between the supply voltage level and 4.7 V below this level. Consequently, a level converter becomes essential for signal processing. Ensure the GND and Test (TE) pins are connected to the Zener diode.