LMC6762

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SNOS739D – JULY 1997 – REVISED MARCH 2013

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.

Output Short Circuit Current

The LMC6762 has short circuit protection of 40 mA. However, 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.

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.

Figure 29. Canceling the Effect of Input Capacitance

The capacitor added across the feedback resistor increases the switching speed and provides more short term

hysteresis. This can result in greater noise immunity for the circuit.

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.

A SPICE macromodel of this and many other op amps is available at no charge from the WEBENCH Design

Center Team at http://www.ti.com/ww/en/analog/webench/

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