ICs for measuring currents usually employ external sense resistors with values of a few tens of milliohms. Not only are such resistors difficult to obtain, the circuit board layout can have a disturbing effect on the accuracy of the measurement. This problem is eliminated by the LM3822 and LM3824 ICs from National Semiconductor (www.national.com), which have built-in sense resistors with a value of only 3 mW. A delta-sigma modulator converts the measured value into a digital value.
A digital filter takes the average value of the current every 50 ms (LM 3822) or every 6 ms (LM 3824). A pulse-width modulated signal (PWM) that is proportional to the current level is generated by comparing this to a digital ramp signal. According to the manufacturer, the LM3822/24 provide the highest measurement accuracy of any currently available high-side current measurement IC. In the case of the LM3824, the accuracy is ±2%. On the output side, the LM3822/24 deliver a pulse-width modulated signal (PWM) whose duty cycle D indicates the measured value of the current, including its sign.
If the current is equal to zero, D is exactly 50 %. Positive currents yield duty cycle values ranging from 50 % to 95.5 %, and negative currents yield duty cycle values ranging from 50 % to 4.5 %. A value of 95.5 % thus corresponds to +1 A (or +2 A), while a value of 4.5 % corresponds to –1 A (or –2 A). The current is considered to be positive when it flows from SENSE to SENSE–.
The LM3822/24 ICs work with a supply voltage between 2.0 V and 5.5 V and have an internal current consumption of less than 150 µA. For operation at voltages greater than 5.5 V, the Zener diode shown in dashed outline in the schematic diagram can be use with a 10-kΩ series resistor. The Zener diode limits the operating voltage across the LM3822/24 to a safe 4.7 V. Note however that in this case, the output signal is not longer referenced to ground, but swings between the supply voltage level and 4.7 V below this level. A level converter is thus needed for signal processing. The GND and Test (TE) pins should be connected to the zener diode.