4

®

REG5608

TYPICAL PERFORMANCE CURVES

APPLICATION INFORMATION

Figure 1 shows a typical SCSI parallel bus termination

circuit. Bypass capacitors should be connected to TERMPWR

and Reg Out pins as shown.

Line capacitance of the REG5608 has been reduced to less

than one-fourth that of the first-generation integrated

terminator ICs. Capacitance difference between package

types and variation by pin number is shown in the typical

performance curve, “Line Capacitance vs Pin Number.” It

shows that the SSOP package provides the lowest capaci-

tance with typical values under 2pF. This variation is

produced primarily by the differences in the interconnection

length to the IC. Of course, actual line capacitance on a

circuit board is generally determined by circuit board layout.

To achieve lowest line capacitance, minimize the total circuit

board interconnection length from the interface cable to the

SCSI transceiver IC and to the REG5608 terminator.

THERMAL CONSIDERATIONS

The REG5608 has internal current limit and thermal

shutdown that protect it from damage in case of output

short-circuit or overload. The current limit is approxi-

mately 550mA and thermal shutdown activates at a

junction temperature of approximately 145

°C. For good

long-term reliability, the junction temperature should not

exceed 125

°C. Any tendency to activate the thermal

shutdown during normal operation is an indication of

inadequate heat sinking and/or excessive power

dissipation.

The terminator's internal power is dissipated primarily by

conduction through the package leads to circuit board traces.

It is important to connect the six thermal ground leads (7, 8,

9, 20, 21, 22) to a large circuit trace—see Figure 2. Typical

measured values of thermal resistance for various circuit

board materials are shown. These are approximate values.

Variations in circuit board pattern, mounting techniques, air

flow, proximity to other circuit boards and heat sources will

affect thermal performance.

A simple experiment will determine whether the actual circuit

board layout is adequate (i.e.,

θ

maximum recommended junction temperature of the REG5608

will not be exceeded. The procedure uses the internal thermal

determine when the junction is approximately 20

°C above the

Operate the circuit with normal or other desired test electrical

conditions. Increase the ambient temperature and determine

the value at which thermal limit occurs (by sensing a sudden

If this occurs at an ambient temperature of more than 20

°C

not exceed 125

°C under the same electrical conditions when

the ambient temperature is at the system design goal value.

TERMINATION CURRENT vs TEMPERATURE

Temperature (°C)

23

22

21

20

19

–25

0

25

50

75

100

125

V

LINE CAPACITANCE vs PIN NUMBER

Pin Number

10

8

6

4

2

0

23456

10

11

12

13

27

26

25

24

23

19

18

17

16

First-Generation Terminator

REG5608U SOIC Package

REG5608E SSOP Package