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PI74FCT162344T for heavy load applications

Posted: 28 Nov 2000     Print Version  Bookmark and Share

Keywords:power 

/ARTICLES/2000NOV/2000NOV28_PL_AN2.PDF

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Application Brief 4

93 01/11/99

PI74FCT162344T for Heavy Load Applications

Application Brief 4

Introduction

Complex memory modules and high-end motherboards require

a driver to buffer and drive many loads. Often, the total capacita-

tive load for each load may exceed 150pF. As a result,

the driver propagation delay significantly increases and the signal

waveform deteriorates as it travels down the trace. This brief

describes the PI74FCT162344T advantages in heavy load

applications.

Enhanced Driving of Transmission Lines

On high-performance motherboards, the signal rise/fall times are

extremely fast, undermining signal integrity. Figure 1 shows a

typical FCT16244 buffer driving 12 logic devices. Since most

traces on the board must be treated as transmission lines, this load-

ing condition causes signal integrity to deteriorate even more.

On the other hand, the PI74FCT162344T has far greater fanout.

Its output stage was designed for balanced a drive (both rise and

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Figure 1. Driver With Heavy Loads

fall directions). The I/V curve indicates 124mA static current.

Actually the available instantaneous current is 150mA which

allows for driving high capacitance loads. Figure 2 exhibits each

of the 4 outputs driving only 3 loads instead of all 12...this is a

major advantage. Thus theAddress/Clock Driver devices offer a

solution for converting heavy loads to light loads.

Maintaining Low Skew

Because there are twice as many output drivers on the FCT162344,

32 to 16 on the FCT16244, skew between drivers becomes very

important. Skew is typically 0.25ns (see data sheet for exact

specification method). Figure 2 exhibits this skew. It should be

noted that trace length matching is also extremely important.

Typically propagation delay is 0.2ns per inch of trace. If traces are

mismatched by 1 inch, this would result in a skew error of 200ps.

Figure 2. Improved Solution - Driver With Nominal Loads

FCT162344

OUT11

OUT12

OUT13

OUT14

IN1

IN2

FCT16244

OUT

IN1

IN2

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Application Brief 4

94 01/11/99

Product Description

Pericom Semiconductor's PI7FCT series of logic circuits are pro-

duced using the company's advanced 0.6 micron CMOS

technology to achieve industry leading speed grades.

The PI74FCT162344T is an address/clock driver designed to

provide fanout to memory arrays. Eight banks, each with a fanout

of four, and 3-state control, provide efficient address distribution.

One or more banks may be used for clock distribution.

The PI74FCT162344T has balanced output drivers. It is designed

with current limiting resistors at its outputs to control the output

edge rate resulting in lower ground bounce and undershoot.

This eliminates the need for external terminating resistors for most

interface applications.

Logic Block Diagram

Product Features

7 Ideal for address line driving and clock distribution

7 Eight banks with 1:4 fanout with 3-State control

7 Typical tsk(o) output skew < 250ps

7 Propagation Delay: 3.8ns Max

7 Balanced output drivers: 124mA

7 Hysteresis on all inputs

7 Packages available:

- 56-pin 240-mil wide plastic TSSOP (A56)

- 56-pin 3000-mil wide plastic SSOP (V56)

Pericom Semiconductor Corporation

2380 Bering Drive 7 San Jose, CA 95131 7 1-800-435-2336 7 Fax (408) 435-1100 7 http://www.pericom.com

A1

B11

OE1

B14

A2

B21

B24

A5

B51

OE3

B54

A6

B61

B64

A3

B31

OE2

B34

A4

B41

B44

A7

B71

OE4

B74

A8

B81

B84





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