AND8020 datasheet(10/18 Pages) AD

10 / 18 page

AND8020/D

http://onsemi.com

Because the resistor divider network of R1 and R2 is used

to generate VTT, the variation in VTT will be intimately tied

to the variation in VCC. Differentiating the equation for VTT

with respect to VCC yields:

DVTT

DVCC

(R1

) R2 )

DVCC

(eq. 24)

For the nominal case, this equation reduces to:

DVTT + 0.6 DVCC

(eq. 25)

DVCC = $5% = $0.25 V, then DVTT = $0.15 V.

As mentioned previously, the real potential for problems

will be if the VOL level can potentially put the output emitter

follower out of the active operating region and into cutoff.

Because of the relationship between the VCC and VTT levels,

the only cutoff risk condition occurs at VCCmin, the lowest

value of VCC. Applying the equation for IOLmin under this

−5% VCC condition yields:

IOLmin +

(VOLmin * VTT )

(eq. 26)

IOLmin +

(4.75

* 1.85) * 2.85

+ 1.0 mA (eq. 27)

The results of this cutoff risk analysis show there is no

potential for the output emitter follower to be in cutoff. This

would indicate a Thevenin equivalent termination scheme is

more robust to variation in VCC. Since the designer has the

flexibility of choosing the VTT level via the selection of the

R1 and R2 resistors, the following procedure can be used.

At −5% minimal variation case for VCC:

VCC = 4.75 V

VTT = VCC − 2.0 V = 2.75 V

R2 = 119

R1 = 86

Thus:

IOHmax = 23 mA

IOLmin = 3.0 mA

At +5% minimal variation case for VCC:

VCC = 5.25 V

VTT = 3.05 V

Thus:

IOHmax = 28 mA

IOLmin = 5.2 mA

Although the output currents are slightly higher than

nominal, the elimination of emitter follower cutoff risk is

well justified.

When the equivalent termination resistance matches the

line impedance, no reflection occurs because all the energy

in the signal is dissipated by the termination. Hence, in

comparing properly terminated schemes parallel and

Thevenin, a primary consideration is the power supply

requirements. As mentioned earlier, the parallel VTT scheme

requires an extra power supply; however, the Thevenin

termination

dissipates

times

power.

Fortunately, this extra power dissipation cannot be seen on

the die; therefore, either technique results in similar die

junction temperatures.

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