LMV7271_15 Datasheet, PDF (14/35 Pages) Texas Instruments – Single and Dual, 1.8-V Low Power Comparators With Rail-to-Rail Input

LMV7271, LMV7272, LMV7275
SNOSA56I – FEBRUARY 2003 – REVISED SEPTEMBER 2015
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Device Functional Modes (continued)
7.4.2 Noise
Most comparators have rather low gain. This allows the output to alternate between high and low when the input
signal changes slowly. The result is the output may oscillate between high and low when the differential input is
near zero and triggers on noise. The high gain of this comparator eliminates this problem. Less than 1 μV of
change on the input will drive the output from one rail to the other rail. If the input signal is noisy, the output
cannot ignore the noise unless some hysteresis is provided by positive feedback. (See Hysteresis.)
7.4.3 Hysteresis
It is a standard procedure to use hysteresis (positive feedback) around a comparator to prevent oscillation due to
the comparator triggering its own noise on slowly ramping signals. The following sections will describe various
ways to apply hysteresis.
7.4.3.1 Noninverting Comparator With Hysteresis
V_CC
V_REF
-
V_A
V_IN
+
R₁
V_O
R_L
R₂
Figure 27. Noninverting Comparator With Hysteresis
A noninverting comparator with hysteresis requires a two resistor network, and a voltage reference (V_REF) at the
inverting input. When V_IN is low, the output is also low. For the output to switch from low to high, V_IN must rise up
to V_IN1 where V_IN1 is calculated by:
'V_IN1
V_REF (R₁ R₂ )
R₂
(1)
As soon as V_O switches to V_CC, V_A steps to a value greater than V_REF which is given by:
V_A
V_IN
(V_CC V_IN1)R₁
R₁ R₂
(2)
To make the comparator switch back to its low state, V_IN must equal V_REF before V_A will again equal V_REF. V_IN2
can be calculated by:
V_IN2
V_REF (R₁ R₂ ) V_CC R₁
R₂
(3)
The hysteresis of this circuit is the difference between V_IN1 and V_IN2.
ΔV_IN = V_CCR₁ / R₂
(4)
Figure 28. Noninverting Comparator Thresholds
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