
MC34058 MC34059
9
MOTOROLA ANALOG IC DEVICE DATA
θ
ja = j, j = 40, 60, .. 100
°
C/W.
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 A
The quiescent power is given by:
Balanced Operation:
To determine the amount of power dissipated by each
output stage we need to know the differential output voltage
for the output current required. Figure 7 shows that for IOH
and IOL differential of 50 mA, VODH and VODL are:
Unbalanced Operation:
To determine the amount of power dissipated by each
output stage we need to know the single–ended output
voltage for the output current required. Figures 5 and 6
shows that for an IOH and IOL of
±
50 mA,
VOH
(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.
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.
PDmax ja, TA
TJmax–TA
ja
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;
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;
VOD
|3.0|, and IOL
|IOH|
IOut
0.050 A.
And the power dissipated by each driver is given by;
PDrvB
PDrvB
IOut·
0.10 W.
VCC– VOD
and equal to
3.9 V
VOL
0.895 V
And the power dissipated by each driver is calcu-
lated by;
PDrvU
and equal to
VCC– VOH
· |IOH|
VOL· IOL
PDrvU
0.10 W.
20
3.0
W
TA (
°
C)
30
40
60
80
90
2.5
1.5
0.5
0
1.0
2.0
* 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.
Graph of Maximum Power Dissipation Possible
for a Particular
θ
ja and Ambient Temperature
70
50
PDTotal
PDmax (
θ
ja40
PQ
ICCQ· VCC, and is equal to PQ
0.09 W.
*SOA
PDmax (
θ
ja60
PDmax (
θ
j80
PDmax (
θ
ja100