LMC7211 Datasheet, PDF (8/13 Pages) National Semiconductor (TI) – Tiny CMOS Comparator with Rail-to-Rail Input and Push-Pull Output

Application Information (Continued)
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FIGURE 2. Circuit for Measurement of the
Shoot-Through Current
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FIGURE 3. Measurement of the Shoot-Through Current
From Figure 3, the shoot-through current for the LMC7211
can be calculated to be 0.2 mA (typical), and the duration is
1 µs. The values needed for the bypass capacitors can be
calculated as follows:
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Area of ∆ = ¹â„2 (1 µs x 200 µA)
= 100 pC
The capacitor needs to supply 100 picocolumb. To avoid
large shifts in the comparator threshold due to changes in
the voltage level, the voltage drop at the bypass capacitor
should be limited to 100 mV or less.
The charge needed (100 picocolumb) and the allowable volt-
age drop (100 mV) will give us the minimum capacitor value
required.
∆Q = C (∆V)
C = ∆Q/∆V = 100 picocolumb/100 mV
C = 10^-10/10^-1 = 10^-9 = 1 nF = 0.001 µF
10^-9 = 1 nF = 0.001 µF
The voltage drop of ∼100 mV will cause a threshold shift in
the comparator. This threshold shift will be reduced by the
power supply rejection ratio, (PSRR). The PSRR which is
applicable here is not the DC value of PSRR (∼80 dB), but a
transient PSRR which will be usually about 20 dB–40 dB,
depending on the circuit and the speed of the transient. This
will result in an effective threshold shift of about 1 mV to
10 mV.
For precision and level sensing circuits, it is generally a good
goal to reduce the voltage delta on the power supply to a
value equal to or less than the hysteresis of the comparator
circuit. If the above circuit was to be used with 50 mV of hys-
teresis, it would be reasonable to increase the bypass ca-
pacitor to 0.01 µF to reduce the voltage delta to 10 mV.
Larger values may be useful for obtaining more accurate and
consistent switching.
Note that the switching current of the comparator can spread
to other parts of the board as noise. The bypass capacitor re-
duces this noise. For low noise systems this may be reason
to make the capacitor larger.
For non-precision circuits, such as using a comparator to de-
termine if a push-button switch is on or off, it is often cheaper
and easier to use a larger value of hysteresis and a small
value or bypass capacitance. The low shoot-through current
of the LMC7211 can allow the use of smaller and less expen-
sive bypass capacitors in non-critical circuits.
4.0 Output Short Circuit Current
The LMC7211 has short circuit protection of 40 mA. How-
ever, it is not designed to withstand continuous short circuits,
transient voltage or current spikes, or shorts to any voltage
beyond the supplies. A resistor in series with the output
should reduce the effect of shorts. For outputs which send
signals off PC boards additional protection devices, such as
diodes to the supply rails, and varistors may be used.
5.0 Hysteresis
If the input signal is very slow or very noisy, the comparator
output might trip several times as the input signal passes
through the threshold. Using positive feedback to add hys-
teresis to the switching can reduce or eliminate this problem.
The positive feedback can be added by a high value resistor
(R_F). This will result in two switching thresholds, one for in-
creasing signals and one for decreasing signals. A capacitor
can be added across R_F to increase the switching speed and
provide more short term hysteresis. This can result in greater
noise immunity for the circuit.
See Figure 4, Figure 5 and Figure 6.
Note that very heavy loading of the comparator output, such
as LED drive or bipolar logic gates, will change the output
voltage and shift the voltage thresholds.
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