LMC7211AIM National Semiconductor, LMC7211AIM Datasheet - Page 9

• Manufacturer Part Number: LMC7211AIM
• Description: Voltage Comparator IC
• Manufacturer: National Semiconductor

Specifications of LMC7211AIM

• No. Of Comparators: 1
• Response Time: 4µs
• No. Of Pins: 8
• Output Type: Push Pull
• Single Supply Voltage Min (+v): 2.7V
• Mounting Type: Surface Mount
• Rail To Rail I/o Type: Rail Rail Inputs
• Number Of Elements: 1
• Technology: CMOS
• Input Offset Voltage: 5mV
• Input Bias Current (typ): 40pA
• Single Supply Voltage (typ): 3/5/9/12V
• Dual Supply Voltage (typ): Not RequiredV
• Supply Current (max): 0.014@15VmA
• Power Supply Requirement: Single
• Common Mode Rejection Ratio: 75dB
• Voltage Gain In Db: 100dB
• Power Supply Rejection Ratio: 80dB
• Single Supply Voltage (min): 2.7V
• Single Supply Voltage (max): 15V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 8
• Package Type: SOIC N
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

FIGURE 1. Even at Low-Supply Voltage of 2.7V, an Input
Signal which Exceeds the Supply Voltages Produces No
At V
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 voltages. Usually,
glitches in the supply lines are prevented by bypass capaci-
tors. When the glitches are minimal, the value of the bypass
capacitors can be reduced.
+
= 2.7V propagation delays are t
FIGURE 2. Circuit for Measurement of the
Phase Inversion at the Output
Shoot-Through Current
PLH
= 4 μs and t
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PHL
= 4
9
From
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
The capacitor needs to supply 100 picocolumb. To avoid large
shifts in the comparator threshold due to changes in the volt-
age 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.
ΔQ
The voltage drop of
the comparator. This threshold shift will be reduced by the
power supply rejection ratio, (PSRR). The PSRR which is ap-
plicable here is not the DC value of PSRR (
transient PSRR which will be usually about 20 dB–40 dB, de-
pending 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.
FIGURE 3. Measurement of the Shoot-Through Current
Figure
= C (ΔV)
C = ΔQ/ΔV = 100 picocolumb/100 mV
C = 10
10
-9
= 1 nF = 0.001 μF
-10
3, the shoot-through current for the LMC7211
/10
-1
= ½ (1 μs × 200 μA)
= 100 pC
∼
= 10
100 mV will cause a threshold shift in
-9
= 1 nF = 0.001 μF
∼
80 dB), but a
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