LMV7235_06 NSC [National Semiconductor], LMV7235_06 Datasheet - Page 11

• Manufacturer Part Number: LMV7235_06
• Description: 45 nsec, Ultra Low Power, Low Voltage, Rail-to-Rail Input Comparator with Open-Drain/Push-Pull Output
• Manufacturer: NSC [National Semiconductor]

Application Information
CIRCUIT LAYOUT AND BYPASSING
The LMV7235/LMV7239 require high speed layout. Follow
these layout guidelines:
1. Use printed circuit board with a good, unbroken low-
inductance ground plane.
2. Place a decoupling capacitor (0.1µF ceramic surface
mount capacitor) as close as possible to V
3. On the inputs and the output, keep lead lengths as short
as possible to avoid unwanted parasitic feedback around the
comparator. Keep inputs away from output.
4. Solder the device directly to the printed circuit board rather
than using a socket.
5. For slow moving input signals, take care to prevent para-
sitic feedback. A small capacitor (1000pF or less) placed
between the inputs can help eliminate oscillations in the
transition region. This capacitor causes some degradation to
t
6. The topside ground plane runs between the output and
inputs.
7. Ground trace from the ground pin runs under the device
up to the bypass capacitor, shielding the inputs from the
outputs.
ZERO-CROSSING DETECTOR
The inverting input is connected to ground and the non-
inverting input is connected to 100mVp-p signal. As the
signal at the non-inverting input crosses 0V, the compara-
tor’s output changes state.
THRESHOLD DETECTOR
Instead of tying the inverting input to 0V, the inverting input
can be tied to a reference voltage. The non-inverting input is
connected to the input. As the input passes the V
old, the comparator’s output changes state.
PD
when the source impedance is low.
FIGURE 3. Zero-Crossing Detector
FIGURE 4. Threshold Detector
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(Continued)
CC
pin.
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REF
thresh-
11
CRYSTAL OSCILLATOR
A simple crystal oscillator using the LMV7239 is shown
below. Resistors R1 and R2 set the bias point at the com-
parator’s non-inverting input. Resistors R3, R4 and C1 sets
the inverting input node at an appropriate DC average level
based on the output. The crystal’s path provides resonant
positive feedback and stable oscillation occurs. The output
duty cycle for this circuit is roughly 50%, but it is affected by
resistor tolerances and to a lesser extent by the comparator
offset.
IR RECEIVER
The LMV7239 is an ideal candidate to be used as an infrared
receiver. The infrared photo diode creates a current relative
to the amount of infrared light present. The current creates a
voltage across R
applied by the voltage divider to the inverting input, the
output transitions.
FIGURE 5. Crystal Oscillator
D
FIGURE 6. IR Receiver
. When this voltage level cross the voltage
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