LMC7211-N_14 Datasheet, PDF (11/23 Pages) Texas Instruments – LMC7211 Tiny CMOS Comparator with Rail-to-Rail Input and Push-Pull Output

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LMC7211-N
SNOS746F – MAY 2004 – REVISED JANUARY 2013
Figure 5. Measurement of the Shoot-Through Current
From Figure 5, 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:
Area of Δ = ½ (1 μs × 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 voltage drop (100 mV) will give us the minimum
capacitor value required.
ΔQ = C (ΔV)
C = ΔQ/ΔV = 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 hysteresis, it would be reasonable to increase the bypass capacitor 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 reduces 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 determine 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 expensive bypass capacitors in non-critical
circuits.
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