參數資料
型號: LMC6762
廠商: National Semiconductor Corporation
英文描述: Dual MicroPower Rail-To-Rail Input CMOS Comparator with Push-Pull Output
中文描述: 雙微功耗軌至與推軌輸入的CMOS比較,推挽輸出
文件頁數: 9/13頁
文件大?。?/td> 317K
代理商: LMC6762
Application Hints
(Continued)
From Figure 3and Figure 4the shoot-through current for the
LMC6762 can be approximated to be 0.2 mA(200 mV/1 k
).
The duration of the transient is measured as 1 μs. The val-
ues needed for the local bypass capacitors can be calcu-
lated as follows:
Area of
=
1
2
(1 μs x 200 μA)
= 100 pC
If the local bypass capacitor has to provide this charge of
100 pC, the minimum value of the local capacitor to prevent
local degradation of V
can be calculated. Suppose that the
maximum voltage droop that the system can tolerate is
100mV,
Q
= C
*
(
V)
C = (
Q/
V)
= 100 pC/100 mV
= 0.001 μF
The low internal feedthrough current of the LMC6762 thus
requires lower values for the local bypass capacitors. In ap-
plications where precision is not critical, this is a significant
advantage, as lower values of capacitors result in savings of
board space, and cost.
It is worth noting here that the delta shift of the power supply
voltage due to the transient currents causes a threshold shift
of the comparator. This threshold shift is reduced by the high
PSRR of the comparator. However, the value of the PSRR
applicable in this instance is the transient PSRR and not the
DC PSRR. The transient PSRR is significantly lower than the
DC PSRR.
Generally, it is a good goal to reduce the delta voltage on the
power supply to a value equal to or less than the hysteresis
of the comparator. For example, if the comparator has 50 mV
of hysteresis, it would be reasonable to increase the value of
the local bypass capacitor to 0.01 μF to reduce the voltage
delta to 10 mV.
4.0 Output Short Circuit Current
The LMC6762 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 is 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 noisy, the comparator output might
trip several times as the input signal repeatedly passes
through the threshold. This problem can be addressed by
making use of hysteresis as shown below.
The capacitor added across the feedback resistor increases
the switching speed and provides more short term hyster-
esis. This can result in greater noise immunity for the circuit.
6.0 Spice Macromodel
A Spice Macromodel is available for the LMC6762. The
model includes a simulation of:
Input common-mode voltage range
Quiescent and dynamic supply current
Input overdrive characteristics
and many more characteristics as listed on the macromodel
disk.
Contact the National Semiconductor Customer Response
Center at 1-800-272-9959 to obtain an operational amplifier
spice model library disk.
Typical Applications
One-Shot Multivibrator
DS012320-8
FIGURE 4. Measurement of the Shoot-Through Current
DS012320-9
DS012320-10
FIGURE 5. Canceling the Effect of Input Capacitance
DS012320-14
FIGURE 6. One-Shot Multivibrator
www.national.com
9
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