LMC7211AIM5X National Semiconductor, LMC7211AIM5X Datasheet - Page 9

• Manufacturer Part Number: LMC7211AIM5X
• Description: IC,VOLT COMPARATOR,SINGLE,CMOS,TSOP,5PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LMC7211AIM5X

• Rohs Compliant: NO

Application Information
This feature, in addition to the comparator being able to
sense signals near the positive rail, is extremely useful in low
voltage applications.
At V
4 µs with overdrives of 100 mV.
Please refer to the performance curves for more extensive
characterization.
3.0 Shoot-Through Current
The shoot-through current is defined as the current surge,
above the quiescent supply current, between the positive
and negative supplies of a device. The current surge occurs
when the output of the device switches states. The
shoot-through current results in glitches in the supply volt-
ages. Usually, glitches in the supply lines are prevented by
bypass capacitors. When the glitches are minimal, the value
of the bypass capacitors can be reduced.
FIGURE 1. Even at Low-Supply Voltage of 2.7V, an
Input Signal which Exceeds the Supply Voltages
+
= 2.7V propagation delays are t
Produces No Phase Inversion at the Output
FIGURE 2. Circuit for Measurement of the
Shoot-Through Current
PLH
= 4 µs and t
(Continued)
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PHL
=
9
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:
Area of ∆ =
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.
∆Q = C (∆V)
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.
FIGURE 3. Measurement of the Shoot-Through Current
C = ∆Q/∆V = 100 picocolumb/100 mV
C = 10
10
-9
= 1 nF = 0.001 µF
= 100 pC
-10
1
⁄
/10
2
(1 µs x 200 µA)
-1
= 10
-9
= 1 nF = 0.001 µF
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