LMC7221

SNOS748E – SEPTEMBER 1999 – REVISED MARCH 2013

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Driving LEDs (Light Emitting Diodes)

With a 5 volt power supply, the LMC7221's output sinking current can drive small, high efficiency LEDs for

indicator and test point circuits. The small size of the Tiny package makes it easy to find space to add this

feature to even compact designs.

Input range to Beyond Rail to Rail

The input common mode range of the LMC7221 is slightly larger than the actual power supply range. This wide

input range means that the comparator can be used to sense signals close to the power supply rails. This wide

input range can make design easier by eliminating voltage dividers, amplifiers, and other front end circuits

previously used to match signals to the limited input range of earlier comparators. This is useful to power supply

monitoring circuits which need to sense their own power supply, and compare it to a reference voltage which is

close to the power supply voltage. The wide input range can also be useful for sensing the voltage drop across a

current sense resistor for battery chargers.

Zero Crossing Detector

Since the LMC7221's common mode input range extends below ground even when powered by a single positive

supply, it can be used with large input resistors as a zero crossing detector.

Low Input Currents and High Input Impedance

These characteristics allow the LMC7221 to be used to sense high impedance signals from sensors. They also

make it possible to use the LMC7221 in timing circuits built with large value resistors. This can reduce the power

dissipation of timing circuits. For very long timing circuits, using high value resistors can reduce the size and cost

of large value capacitors for the same R-C time constant.

Direct Sensor Interfacing

The wide input voltage range and high impedance of the LMC7221 may make it possible to directly interface to a

sensor without the use of amplifiers or bias circuits. In circuits with sensors which can produce outputs in the

tens to hundreds of millivolts, the LMC7221 can compare the sensor signal with an appropriately small reference

voltage. This may be done close to ground or the positive supply rail. Direct sensor interfacing may eliminate the

need for an amplifier for the sensor signal. Eliminating the amplifier can save cost, space, and design time.

LOW VOLTAGE OPERATION

Comparators are the common devices by which analog signals interface with digital circuits. The LMC7221 has

been designed to operate at supply voltages of 2.7V without sacrificing performance to meet the demands of 3V

digital systems.

At supply voltages of 2.7V, the common-mode voltage range extends 200 mV (ensured) below the negative

supply. This feature, in addition to the comparator being able to sense signals near the positive rail, is extremely

useful in low voltage applications.

Figure 16. Even at Low-Supply Voltage of 2.7V, an Input Signal which Exceeds the Supply Voltages

Produces No Phase Inversion at the Output

Please refer to the performance curves for more extensive characterization.

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