AN-90 データシートの表示（PDF） - Fairchild Semiconductor

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AN-90

The Use of QFETs in a Flyback Converter

Fairchild Semiconductor 

fanout for the 4000 series while 54C/74C has a fanout of

40 pF. A fanout of 5 pF (one gate input) is all but useless, and

specified propagation delay would most probably not be re-

alized in an actual system.

The same formula and curves may be applied to more com-

plex devices. For example the propagation delay from data

to output for an MM74C157 operating at VCC = 10V and CL

= 100 pF is:

AN006021-10

FIGURE 7. Typical Propagation Delay per pF of Load

Capacitance vs Power Supply

Operating at loads other than 50 pF poses a problem since

propagation is a function of load capacitance. To simplify the

problem Figure 7 has been generated and gives the slope of

the propagation delay vs load capacitance line (∆tpd/pF) as a

function of power supply voltage. Because the propagation

delay for zero load capacitance is not zero and depends on

the internal structure of each device, an offset term must be

added that is unique to a particular device type. Since each

data sheet gives propagation delay for 50 pF the actual de-

lay for different loads can be computed with the aid of the fol-

lowing equation:

It is significant to note that this equation and Figure 7 apply

to all 54C/74C devices. This is true because of the close

match in drive characteristics of every device including MSI

functions, i.e., the slope of the propagation delay vs load ca-

pacitance line at a given voltage is typically equal for all de-

vices. The only exception is high fan-out buffers which have

a smaller ∆tpd/pF.

Another point to consider in the design of a CMOS system is

the effect of power supply voltage on propagation delay. Fig-

ure 8 shows propagation delay as a function of VCC and

propagation delay times power consumption vs VCC for an

MM74C00 operating with 50 pF load at f = 100 kHz.

As an example let’s compute the propagation delay for an

MM74C00 driving 15 pF load and operating with a

VCC = 5.0V. The equation gives:

AN006021-11

FIGURE 8. Speed Power Product and

Propagation Delay vs VCC

Above VCC = 5.0V note the speed power product curve ap-

proaches a straight line. However the tpd curve starts to “flat-

ten out”. Going from VCC = 5.0V to VCC = 10V gives a 40%

decrease in propagation delay and going from VCC = 10V to

VCC = 15V only decreases propagation delay by 25%.

Clearly for VCC > 10V a small increase in speed is gained by

a disproportionate increase in power. Conversely, for small

decreases in power below VCC = 5.0V large increases in

propagation delay result.

Obviously it is optimum to use the lowest voltage consistent

with system speed requirements. However, in general it can

be seen from Figure 8 that the best speed power perfor-

mance will be obtained in the VCC = 5.0V to VCC = 10V

range.

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