sm72375mmx National Semiconductor Corporation, sm72375mmx Datasheet - Page 8

• Manufacturer Part Number: sm72375mmx
• Description: Solarmagic Dual Micropower Rail-to-rail Input Cmos Comparator With Open Drain Output
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
1.0 INPUT COMMON-MODE VOLTAGE RANGE
At supply voltages of 2.7V, 5V and 15V, the SM72375 has an
input common-mode voltage range (CMVR) which exceeds
both supplies. As in the case of operational amplifiers, CMVR
is defined by the V
mon-mode range of the device. A common-mode rejection
ratio (CMRR, defined as ΔV
plies a shift of < 1 mV over the entire common-mode range of
the device. The absolute maximum input voltage at V
is 200 mV beyond either supply rail at room temperature.
A wide input voltage range means that the comparator can be
used to sense signals close to ground and also to the power
supplies. This is an extremely useful feature in power supply
monitoring circuits.
An input common-mode voltage range that exceeds the sup-
plies, 20 fA input currents (typical), and a high input
impedance makes the SM72375 ideal for sensor applications.
The SM72375 can directly interface to sensors without the
use of amplifiers or bias circuits. In circuits with sensors which
produce outputs in the tens to hundreds of millivolts, the
SM72375 can compare the sensor signal with an appropri-
ately small reference voltage. This reference voltage can be
close to ground or the positive supply rail.
2.0 LOW VOLTAGE OPERATION
Comparators are the common devices by which analog sig-
nals interface with digital circuits. The SM72375 has been
designed to operate at supply voltages of 2.7V, without sac-
rificing performance, to meet the demands of 3V digital sys-
tems.
At supply voltages of 2.7V, the common-mode voltage range
extends 200 mV (guaranteed) 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 1. An Input Signal Exceeds the SM72375 Power
Supply Voltages with No Output Phase Inversion
OS
shift of the comparator over the com-
OS
/ΔV
CM
) of 75 dB (typical) im-
30141824
+
= 5V
8
FIGURE 2. Even at Low-Supply Voltage of 2.7V, an Input
Signal which Exceeds the Supply Voltages Produces No
At V
μs with overdrives of 100 mV. Please refer to the performance
curves for more extensive characterization.
3.0 OUTPUT SHORT CIRCUIT CURRENT
The SM72375 has short circuit protection of 40 mA. However,
it is not designed to withstand continuous short circuits, tran-
sient voltage or current spikes, or shorts to any voltage be-
yond the supplies. A resistor is series with the output should
reduce the effect of shorts. For outputs which send signals off
PC boards additional protection devices, such as diodes to
the supply rails, and varistors may be used.
4.0 HYSTERESIS
If the input signal is very noisy, the comparator output might
trip several times as the input signal repeatedly passes
through the threshold. This problem can be addressed by
making use of hysteresis as shown below.
The capacitor added across the feedback resistor increases
the switching speed and provides more short term hysteresis.
This can result in greater noise immunity for the circuit.
FIGURE 3. Canceling the Effect of Input Capacitance
+
= 2.7V, propagation delays are t
Phase Inversion at the Output
PLH
= 4 μs and t
30141825
PHL
30141826
= 4