Datasheet

LMP7300
www.ti.com SNOSAT7F –AUGUST 2007–REVISED MARCH 2013
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 compensate 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 reference 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 threshold 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 maintaining 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 micropower applications.
Fault Detection Rate
The user’s choice of a pullup resistor and capacitive load determines the minimum response time and the event
detection rate. By optimizing overdrive, the pullup resistor and capactive 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. 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 2.7V. For the typical example, a differential input signal voltage, V
IN
, is applied between INP and
INN, the non-inverting 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 14 through Figure 22 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.
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