lmc6762bim National Semiconductor Corporation, lmc6762bim Datasheet - Page 10

• Manufacturer Part Number: lmc6762bim
• Description: Dual Micropower Rail-to-rail Input Cmos Comparator With Push-pull Output
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Hints
From Figure 3 and Figure 4 the shoot-through current for the
LMC6762 can be approximated to be 0.2 mA (200 mV/1 kΩ).
The duration of the transient is measured as 1 µs. The
values needed for the local bypass capacitors can be calcu-
lated as follows:
Area of ∆ =
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
maximum voltage droop that the system can tolerate is
100mV,
∆Q = C * (∆V)
The low internal feedthrough current of the LMC6762 thus
requires lower values for the local bypass capacitors. In
applications where precision is not critical, this is a signifi-
cant advantage, as lower values of capacitors result in sav-
ings 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
→ C = (∆Q/∆V)
FIGURE 4. Measurement of the Shoot-Through Current
= 100 pC/100 mV
= 0.001 µF
= 100 pC
1
⁄
2
(1 µs x 200 µA)
CC
can be calculated. Suppose that the
(Continued)
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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.
FIGURE 5. Canceling the Effect of Input Capacitance
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