LMV7271_08 NSC [National Semiconductor], LMV7271_08 Datasheet - Page 12

• Manufacturer Part Number: LMV7271_08
• Description: Single & Dual, 1.8V Low Power Comparators with Rail-to-Rail Input
• Manufacturer: NSC [National Semiconductor]

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CIRCUIT TECHNIQUES FOR AVOIDING OSCILLATIONS
IN COMPARATOR APPLICATIONS
Feedback to almost any pin of a comparator can result in os-
cillation. In addition, when the input signal is a slow voltage
ramp or sine wave, the comparator may also burst into oscil-
lation near the crossing point. To avoid oscillation or instabil-
ity, PCB layout should be engineered thoughtfully. Several
precautions are recommended:
1. Power supply bypassing is critical, and will improve sta-
2. Keep all leads short to reduce stray capacitance and lead
3. It is a good practice to use an unbroken ground plane on
FIGURE 4. Non-Inverting Comparator with Hysteresis
bility 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 ad-
jacent 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 applica-
tion, a 0.1μF capacitor is recommended to be placed
across V
inductance. It will also minimize any unwanted coupling
from any high-level signals (such as the output). The com-
parators can easily oscillate if the output lead is inadver-
tently 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).
a printed circuit board to provide all components with a low
inductive ground connection. Make sure ground paths are
+
and V
−
pins.
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4. The output trace should be routed away from inputs. The
5. When the signal source is applied through a resistive net-
6. All pins of any unused comparators should be tied to the
micro SMD LIGHT SENSITIVITY
Exposing the micro SMD device to direct sunlight will cause
mis-operation of the device. Light sources such as Halogen
lamps can also affect electrical performance if brought near
to the device. The wavelengths, which have the most detri-
mental effect, are reds and infrareds.
micro SMD MOUNTING
The micro SMD package requires specific mounting tech-
niques, which are detailed in National Semiconductor Appli-
cation Note AN-1112.
LMV7272 micro SMD to DIP Conversion Board
To facilitate characterization and testing, a micro SMD to DIP
conversion board, LMV7272TLCONV, is available. It is a 2-
layer board, with the LMV7272 mounted on the bottom layer,
and a capacitor (C1, between the positive and negative sup-
plies) added to the top layer.
low-impedance where heavier currents are flowing to
avoid ground level shift. Preferably there should be a
ground plane under the component.
ground plane should extend between the output and in-
puts to act as a guard. This can be achieved by running a
topside ground plane between the output and inputs. A
typical PCB layout is shown in Figure 5.
work to one input of the comparator, it is usually advan-
tageous to connect the other input with a resistor with the
same value, for both DC and AC consideration. Input
traces should be laid out symmetrically if possible.
negative supply.
FIGURE 5. Typical PCB Layout
LMV7272TLCONV Diagram
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