LMV7291 Datasheet, PDF (13/23 Pages) National Semiconductor (TI) – Single 1.8V Low Power Comparator with Rail-to-Rail Input

LMV7291
www.ti.com
SNOSA86E – FEBRUARY 2004 – REVISED MARCH 2013
When V_IN is high, the output is also high. To make the comparator switch back to its low state, V_IN must equal
V_REF before V_A will again equal V_REF. V_IN can be calculated by:
(5)
The hysteresis of this circuit is the difference between V_IN1 and V_IN2.
ΔV_IN = V_CCR₁/R₂
(6)
Figure 28. Non-Inverting Comparator with Hysteresis
CIRCUIT TECHNIQUES FOR AVOIDING OSCILLATIONS IN COMPARATOR APPLICATIONS
Feedback to almost any pin of a comparator can result in oscillation. In addition, when the input signal is a slow
voltage ramp or sine wave, the comparator may also burst into oscillation near the crossing point. To avoid
oscillation or instability, PCB layout should be engineered thoughtfully. Several precautions are recommended:
1. Power supply bypassing is critical, and will improve stability and transient response. Resistance and
inductance from power supply wires and board traces increase power supply line impedance. When supply
current changes, the power supply line will move due to its impedance. Large enough supply line shift will
cause the comparator to mis-operate. To avoid problems, a small bypass capacitor, such as 0.1uF ceramic,
should be placed immediately adjacent to the supply pins. An additional 6.8μF or greater tantalum capacitor
should be placed at the point where the power supply for the comparator is introduced onto the board. These
capacitors act as an energy reservoir and keep the supply impedance low. In dual supply application, a
0.1μF capacitor is recommended to be placed across V⁺ and V^− pins.
2. Keep all leads short to reduce stray capacitance and lead inductance. It will also minimize any unwanted
coupling from any high-level signals (such as the output). The comparators can easily oscillate if the output
lead is inadvertently allowed to capacitively couple to the inputs via stray capacitance. This shows up only
during the output voltage transition intervals as the comparator changes states. Try to avoid a long loop
which could act as an inductor (coil).
3. It is a good practice to use an unbroken ground plane on a printed circuit board to provide all components
with a low inductive ground connection. Make sure ground paths are low-impedance where heavier currents
are flowing to avoid ground level shift. Preferably there should be a ground plane under the component.
4. The output trace should be routed away from inputs. The ground plane should extend between the output
and inputs to act as a guard.
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