LMP7300_15 Datasheet, PDF (11/30 Pages) Texas Instruments – Micropower Precision Comparator and Precision Reference

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LMP7300
SNOSAT7G – AUGUST 2007 – REVISED OCTOBER 2015
Feature Description (continued)
7.3.3 Hysteresis
False triggering on noise coupled into the signal path is a common problem for comparator based threshold
detectors. One of the most effective solutions is to add hysteresis. Hysteresis is a circuit signal path
characteristic where an amplitude delay is introduced to the normal input. Positive hysteresis forces the signal to
pass the normal switch point before the output makes a low to high transition while negative hysteresis does the
opposite. This is a memory effect. The comparator behaves differently based on which direction the signal is
going.
The LM7300 has been designed with a unique way of introducing hysteresis. The set points are completely
independent of each other, the power supply, and the input or output conditions. The HYSTP pin sets positive
hysteresis and the HYSTN pin sets the negative hysteresis in a simple way using two resistors. The pins can be
tied together for the same hysteresis or tied to separate voltage taps for asymmetric hysteresis, or tied to the
reference for no hysteresis. When the precision reference is used to drive the voltage tap resistor divider precise,
stable threshold levels can be obtained. The maximum recommended hysteresis is about 130 mV. This places
the HYSTP and HYSTN pin voltages at V_REF – 130 mV, which is approximately the center of their input common
mode range at 2.7 V. For the typical example, a differential input signal voltage, V_IN, is applied between INP and
INN, the noninverting and inverting inputs of the comparator. A DC switch or threshold voltage, V_TH, is set on the
negative input to keep the output off when the signal is above and on when it goes below this level. For a
precision threshold tie the INN pin to V_REF. With the output, off the circuit is in the minimum power state.
Figure 13 through Figure 21 demonstrate the different configurations for setting the upper threshold V_IH and the
lower threshold V_IL and their relationship to the input trip point V_REF, by the following formulas.
V_IL
=
V_REF
-
V_REF
æ
ç
è
R₁
R₁ + R₂
ö
÷
ø
V_IH
=
V_REF
+
V_REF
æ
ç
è
R₁
R₁ + R₂
ö
÷
ø
(1)
V⁺
+ VIN
-
R_P1
1.47 k:
1%
R_P2
1 M:
1%
INP
INN
HYSTP
HYSTN
V_REF
R_N1
4.91 k:
1%
R_N2
1 M:
1%
+
-
GND _V+
2.048V
GND
R_PULLUP
1 M:
OUT
Figure 12. Typical Micropower Application to Set Asymmetric Positive and
Negative Hysteresis of −10 mV, 3 mV
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