EL5181CS datasheet(9/11 Pages) ELANTEC

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EL5181C

8ns High-Speed Comparator

times greater than the input bias current of the

comparator:

Let the hysteresis be VH, and calculate R3:

where:

VO=VSD-0.8V (swing of the output)

Recalculate R2 to maintain the same value of VTH:

Non inverting comparator with hysteresis:

R3 adds a portion of the output to the positive input.

Note that the current through R3 should be much greater

than the input bias current in order to minimize errors.

The calculation of the resistor values as follows:

Pick the value of R1. R1 should be small (less than 1kΩ)

in order to minimize the propagation delay time.

Choose the hysteresis VH and calculate R3:

Check the current through R3 and make sure that it is

much greater than the input bias current as follows:

The above two methods will generate hysteresis of up to

a few hundred millivolts. Beyond that, the impedance of

R3 is low enough to affect the bias string and adjustment

of R1 may be required.

Power Dissipation

When switching at high speeds, the comparator's drive

capability is limited by the rise in junction temperature

caused by the internal power dissipation. For reliable

operation, the junction temperature must be kept below

TJMAX (125°C).

An approximate equation for the device power dissipa-

tion is as follows. Assume the power dissipation in the

load is very small:

where:

VS is the analog supply voltage from VS+ to VS-

IS is the analog quiescent supply current per comparator

VSD is the digital supply voltage from VSD to ground

ISD is the digital supply current per comparator

ISD strongly depends on the input switching frequency.

Please refer to the performance curve to choose the input

driving frequency. Having obtained the power dissipa-

tion, the maximum junction temperature can be

determined as follows:

where:

TMAX is the maximum ambient temperature

θJA is the thermal resistance of the package

Threshold Detector

The inverting input is connected to a reference voltage

and the non-inverting input is connected to the input. As

the input passes the VREF threshold, the comparator's

REF

-------------------

--------R

(

2 )

REF

(

TH )

-----------







–

0.5V

–

-------------------------------------







VIN

VREF

(

0.8

)

--------

–

0.5V

REF

–

----------------------------------------

DISS

SD )

(

JMAX

MAX

DISS

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