VCC

VO

R2

R1

VA

+

-

VREF

RL

VIN

18

LPV7215

SNOSAI6J – SEPTEMBER 2005 – REVISED AUGUST 2016

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Device Functional Modes (continued)

Figure 32. Noninverting Comparator With Hysteresis

Figure 33. Noninverting Comparator With Hysteresis

7.4.6 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-mVPP AC signal. As the signal at the noninverting input crosses 0 V, the comparator’s output

changes state.

Figure 34. Zero Crossing Detector

To improve switching times and to center the input threshold to ground a small amount of positive feedback is

added to the circuit. The 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, is satisfied when VIN = 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.