MC34058FTA Ver la hoja de datos (PDF) - Motorola => Freescale

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componentes Descripción

Fabricante

MC34058FTA

Hex EIA–485 Transceiver with Three–State Outputs

Motorola => Freescale 

MC34058 MC34059

Example 1. Balanced and Unbalanced Operation

IOL = 50 mA and IOH = ± 50 mA for each driver. VCC = 5.0 V.

How many drivers can be used? (Typical power supply current ICCQ = 18 mA.)

Solution:

+ + ICCQ = 0.018

The quiescent

A

power

is

given

by:

PQ

ICCQ · VCC, and is equal to PQ

0.09 W.

Balanced Operation:

Unbalanced Operation:

To determine the amount of power dissipated by each

To determine the amount of power dissipated by each

output stage we need to know the differential output voltage

output stage we need to know the single–ended output

for the output current required. Figure 7 shows that for IOH

and IOL differential of 50 mA, VODH and VODL are:

+ + + + VOD |3.0|, and IOL |IOH| IOut 0.050 A.

+ ǒ Ǔ And the power dissipated by each driver is given by;

PDrvB IOut · VCC – VOD and equal to

+ PDrvB 0.10 W.

voltage for the output current required. Figures 5 and 6

shows that for an IOH and IOL of ± 50 mA,

+ + VOH 3.9 V VOL 0.895 V

+ ǒ Ǔ ) And the power dissipated by each driver is calcu-

lated by;

PDrvU VCC – VOH · |IOH| VOL · IOL

+ and equal to

PDrvU 0.10 W.

(For this example, balanced operation is assumed.)

Summing the quiescent and driver power for 6 transceivers operating in a package produces;

PDTotal = PQ + 6 ⋅ PDrvB, and equal to PDTotal = 0.69 W.

For the MC34058/9, the thermal resistance is capable of a wide range. The ability of the package to dissipate power depends

on board type and temperature, layout and ambient temperature (see Appendix A). For the purposes of this example the

thermal resistance can range from 40°C/W to 100°C/W;

θja = j, j = 40, 60, .. 100°C/W.

Varying the ambient operating temperature TA = 25, 30, .. 85°C; specifying a maximum junction temperature to avoid

thermal shutdown TJmax = 130°C; and using the first order approximation for maximum power dissipation;

ǒ Ǔ + PDmax qja , TA

TJmax–TA

qja

produces a set curves that can be used to determine a Safe Operating Area for the specific application. PDTotal is graphed with

PDmax to provide a reference.

Graph of Maximum Power Dissipation Possible

for a Particular θja and Ambient Temperature

3.0

PDmax (θja40), TA

2.5

2.0 PDmax (θja60), TA

1.5

PDmax (θja80), TA

1.0

*SOA

0.5

PDTotal

PDmax (θja100), TA

0

20

30

40

50

60

70

80

90

TA (°C)

* Safe Operating Area (SOA), is an operating power, PDTotal, less than PDmax.

So all the drivers in the package can be used if the thermal resistance and/or the ambient temperature is low enough.

MOTOROLA ANALOG IC DEVICE DATA

9

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