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

= 100 pC

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 volt-

age drop (100 mV) will give us the minimum capacitor value

required.

∆Q = C(∆V)

C =

∆Q/∆V = 100 picocolumb/100 mV

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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 hys-

teresis, it would be reasonable to increase the bypass ca-

pacitor 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 re-

duces this noise. For low noise systems this may be reason

to make the capacitor larger.

For non-precision circuits, such as using a comparator to de-

termine if a push-button switch is on or off, it is often cheaper

and easier to use a larger value of hysteresis and a small

value or bypass capacitance. The low shoot-through current

of the LMC7211 can allow the use of smaller and less expen-

sive bypass capacitors in non-critical circuits.

4.0 Output Short Circuit Current

The LMC7211 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 in 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 slow or very noisy, the comparator

output might trip several times as the input signal passes

through the threshold. Using positive feedback to add hys-

teresis to the switching can reduce or eliminate this problem.

The positive feedback can be added by a high value resistor

(R

creasing signals and one for decreasing signals. A capacitor

can be added across R

provide more short term hysteresis. This can result in greater

noise immunity for the circuit.

See

Figure 4, Figure 5 and Figure 6.

Note that very heavy loading of the comparator output, such

as LED drive or bipolar logic gates, will change the output

voltage and shift the voltage thresholds.

DS012337-6

FIGURE 2. Circuit for Measurement of the

Shoot-Through Current

DS012337-7

FIGURE 3. Measurement of the Shoot-Through Current

DS012337-8

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