lmp7300max National Semiconductor Corporation, lmp7300max Datasheet - Page 8

• Manufacturer Part Number: lmp7300max
• Description: Micropower Precision Comparator And Precision Reference With Adjustable Hysteresis
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
GENERAL DESCRIPTION
The LMP7300 is a unique combination of micropower and
precision. The open collector comparator has low offset, high
CMRR, high PSRR, programmable hysteresis and microamp
supply current. The precision 2.048V reference provides a
DAC or ADC with an accurate binary divisible voltage. The
comparator and reference combination forms an ideal single
IC solution for low power sensor or portable applications.
VOLTAGE REFERENCE
The reference output voltage is a band gap derived 2.048V
that is trimmed to achieve typically 0.2% accuracy over the
full operating temperature range of −40°C to 125°C. The trim
procedure employs a curvature correction algorithm to com-
pensate for the base emitter thermal nonlinearity inherent in
band gap design topologies. The reference accuracy and the
set resistor tolerance determine the magnitude and precision
of the programmable hysteresis. In situations where refer-
ence noise filtering is required a 5 µF capacitor in series with
a 190Ω resistor to ground are recommended.
COMPARATOR
Output Stage
The comparator employs an open collector output stage that
can switch microamp loads for micropower precision thresh-
old detection to applications requiring activating a solenoid, a
lamp, or an LED. The wired-OR type output easily interfaces
to TTL, CMOS, or multiple outputs, as in a window comparator
application, over a range of 0.5V to 12V. The output is capable
of driving greater than 10 mA output current and yet main-
taining a saturation voltage below 0.4V over temperature. The
supply current increases linearly when driving heavy loads so
a pullup resistor of 100 kΩ or greater is recommended for mi-
cropower applications.
Fault Detection Rate
The user’s choice of a pullup resistor and capacitive load de-
termines the minimum response time and the event detection
rate. By optimizing overdrive, the pullup resistor and ca-
pactive load fault update rates of 200 kHz to 250 kHz or
greater can be achieved.
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. Hys-
teresis is a circuit signal path characteristic where an ampli-
tude 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 intro-
ducing hysteresis. The set points are completely independent
of each other, the power supply, and the input or output con-
ditions. 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 hys-
teresis or tied to separate voltage taps for asymmetric hys-
teresis, 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
8
places the HYSTP and HYSTN pin voltages at V
which is approximately the center of their input common mode
range at 2.7V. For the typical example, a differential input
signal voltage, V
inverting and inverting inputs of the comparator. A DC switch
or threshold voltage, V
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
state. Figure 1 through Figure 5 demonstrate the different
configurations for setting the upper threshold V
er threshold V
V
When V
REF
REF
FIGURE 1. Typical Micropower Application to Set
. With the output, off the circuit is in the minimum power
, by the following formulas.
ID
Negative Hysteresis of −10 mV, +3 mV
= 0, INN = INP = V
IL
IN
and their relationship to the input trip point
Asymmetric Positive and
, is applied between INP and INN, the non-
TH
, is set on the negative input to keep
(a)
(b)
TH
IH
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REF
and the low-
– 130 mV
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