LMV7271_15 Datasheet, PDF (16/35 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
Device Functional Modes (continued)
7.4.4 Zero Crossing Detector
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Figure 30. Simple Zero Crossing Detector
In a zero crossing detector circuit, the inverting input is connected to ground and the noninverting input is
connected to a 100 mV_PP AC signal. As the signal at the noninverting input crosses 0 V, the output of the
comparator changes state.
7.4.4.1 Zero Crossing Detector With Hysteresis
V_CC
R3
R₄
R₁
R₂
V_IN
-
V₂
V_O
D1
V₁
+
R₆
R₅
Figure 31. Zero Crossing Detector With Hysteresis
To improve switching times and centering the input threshold to ground a small amount of positive feedback is
added to the circuit. Voltage divider R₄ and R₅ establishes a reference voltage, V₁, at the positive input. By
making the series resistance, R₁ plus R₂ equal to R₅, the switching condition, V₁ = V₂, will be satisfied when V_IN
= 0.
The positive feedback resistor, R₆, is made very large with respect to R₅ || R₆ = 2000 R₅). The resultant
hysteresis established by this network is very small (ΔV₁ < 10 mV) but it is sufficient to insure rapid output
voltage transitions.
Diode D₁ is used to insure that the inverting input terminal of the comparator never goes below approximately
−100 mV. As the input terminal goes negative, D₁ will forward bias, clamping the node between R₁ and R₂ to
approximately −700 mV. This sets up a voltage divider with R₂ and R₃ preventing V₂ from going below ground.
The maximum negative input overdrive is limited by the current handling ability of D₁.
7.4.5 Threshold Detector
Figure 32. Threshold Detector
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