LMP7300MA NSC [National Semiconductor], LMP7300MA Datasheet - Page 11

• Manufacturer Part Number: LMP7300MA
• Description: Micropower Precision Comparator and Precision Reference with Adjustable Hysteresis
• Manufacturer: NSC [National Semiconductor]

In the general case, as demonstrated with both positive and
negative hysteresis bands in Figure 6, noise within these
bands will have no affect on the state of the comparator out-
put. In Example #1 the noise is well behaved and in band. The
output is clean and well behaved. In Example #2, a significant
amount of out of band noise is present but due to hysteresis
no false triggers occur on the rising positive or falling negative
edges. The hysteresis forces the signal level to move higher
or lower before the output is set to the opposite state.
How Much Hysteresis Is Correct?
An effective way of determining the minimum hysteresis nec-
essary for clean switching is to decrease the amount of hys-
teresis until false triggering is observed, and then use a
multiple of say three times that amount of hysteresis in the
final circuit. This is most easily accomplished in the bread-
FIGURE 6. Output Response with Input Noise Less than
FIGURE 7. Temperature Controlled Window Detector to Monitor Ambient Temperature
Hysteresis Band
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board phase by making R
cations near or above +100°C a minimum of 5 mV hysteresis
is recommended due to peaking of the LMP7300 noise sen-
sitivity at high temperatures.
LAYOUT RECOMMENDATIONS
A good PCB layout is always important to reduce output to
input coupling. Positive feedback noise reduces performance.
For the LMP7300 output coupling is minimized by the unique
package pinout. The output is kept away from the non-invert-
ing and inverting inputs, the reference and the hysteresis pins.
EVALUATION BOARDS
National Semiconductor provides the following PCB boards
as an aid in evaluating the LMP7300 performance.
WINDOW COMPARATOR
Figure 8 shows two LMP7300s configured as a micropower
window detector in a temperature level detection application.
The circuit shown monitors the ambient temperature change.
If the temperature rises outside the 15°C to 35°C window, ei-
ther comparator 1 for high temp, or comparator 2 for low temp,
will set low, indicating a fault condition has occurred. The
open collector outputs are pulled up separately but can be
wire-OR’d for a single fault indication. If the temperature re-
turns inside the window it must overcome the 22 mV asym-
metric hysteresis band established on either comparator. For
the high side the temperature must drop below 34°C and for
the low side the temperature must rise above 16°C for the
outputs to reset high and remove the fault indication. The
temperature is sensed by a 30 kΩ @ 25°C Omega Precision
NTC Thermistor #44008 (±0.2% tol).
Device
LMP7300MA
LMP7300MM
Package
8-Pin SOIC
8-Pin MSOP
1
and R
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2
potentiometers. For appli-
Evaluation Board
Ordering ID
LMP7300MA-EVAL
LMP7300MM-EVAL
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