LM2903 datasheet(9/12 Pages) 3PEAK

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LM2903/LM2901

36V Low Power Dual/Quad Differential Comparators

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Rev. B

4. Solder the device directly to the PCB rather than using a socket.

5. For slow-moving input signals, take care to prevent parasitic feedback. A small capacitor (1000 pF or less)

placed between the inputs can help eliminate oscillations in the transition region. This capacitor causes some

degradation to propagation delay when the impedance is low. The topside ground plane should be placed

between the output and inputs.

6. The ground pin ground trace should run under the device up to the bypass capacitor, thus shielding the inputs

from the outputs.

Typical Applications

IR Receiver

The LM2903/2901 is an ideal candidate to be used as an infrared receiver shown in Figure 6. The infrared photo

diode creates a current relative to the amount of infrared light present. The current creates a voltage across RD.

When this voltage level cross the voltage applied by the voltage divider to the inverting input, the output

transitions. Optional Ro provides additional hysteresis for noise immunity.

VDD

LM393

Figure 6. IR Receiver

Relaxation Oscillator

A relaxation oscillator using LM2903/2901 is shown in Figure 7. Resistors R1 and R2 set the bias point at the

comparator's inverting input. The period of oscillator is set by the time constant of R4 and C1. The maximum

frequency is limited by the large signal propagation delay of the comparator. LM2903/2901

’s low propagation

delay guarantees the high frequency oscillation.

If the inverted input (VC1) is lower than the non-inverting input (VA), the output is high which charges C1 through R4

until VC1 is equal to VA. The value of VA at this point is







At this point the comparator switches pulling down the output to the negative rail. The value of VA at this point is







If R1=R2=R3, then VA1=2VDD /3, and VA2= VDD/3

The capacitor C1 now discharges through R4, and the voltage VC decreases till it is equal to VA2, at which point the

comparator switches again, bringing it back to the initial stage. The time period is equal to twice the time it takes

to discharge C1 from 2VDD/3 to VDD/3. Hence the frequency is:

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