Datasheet

V
CC
V
O
R
2
R
1
V
A
+
-
V
REF
R
L
V
IN
V
IN2
=
V
REF
(R
1
+ R
2
) ± V
CC
R
1
R
2
R
1
+ R
2
V
A
= V
IN
+
(V
CC
-V
IN1
) R
1
LPV7215
www.ti.com SNOSAI6I –SEPTEMBER 2005–REVISED APRIL 2013
To make the comparator switch back to it’s low state, V
IN
must equal V
REF
before V
A
will again equal V
REF
. V
IN2
can be calculated by
The hysteresis of this circuit is the difference between V
IN1
and V
IN2
.
ΔV
IN
= V
CC
R
1
/R
2
Figure 35. Non-Inverting Comparator with Hysteresis
ZERO CROSSING DETECTOR
In a zero crossing detector circuit, the inverting input is connected to ground and the non-inverting input is
connected to a 100 mV
PP
AC signal. As the signal at the non-inverting input crosses 0V, the comparator’s output
changes state.
Figure 36. 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, R
4
and R
5
, establishes a reference voltage, V
1
, at the positive input. By
making the series resistance, R
1
plus R
2
equal to R
5
, the switching condition, V
1
= V
2
, will be satisfied when V
IN
= 0. The positive feedback resistor, R
6
, is made very large with respect to R
5
(R
6
= 2000 R
5
). The resultant
hysteresis established by this network is very small (ΔV
1
< 10 mV) but it is sufficient to insure rapid output
voltage transitions. Diode D
1
is used to insure that the inverting input terminal of the comparator never goes
below approximately −100 mV. As the input terminal goes negative, D
1
will forward bias, clamping the node
between R
1
and R
2
to approximately −700 mV. This sets up a voltage divider with R
2
and R
3
preventing V
2
from
going below ground. The maximum negative input overdrive is limited by the current handling ability of D
1
.
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