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參數(shù)資料
| 型號(hào)： | L8551 |
| 廠商： | Lineage Power |
| 英文描述： | Low-Power SLIC(低功耗用戶線接口（SLIC）) |
| 中文描述： | 低功耗用戶接口（低功耗用戶線接口電路（SLIC）） |
| 文件頁數(shù)： | 27/32頁 |
| 文件大小： | 526K |
| 代理商： | L8551 |
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Lucent Technologies Inc.
27
Data Sheet
March 1997
L8551 Low-Power SLIC
Applications
(continued)
Power Derating
Thermal considerations can affect the choice of 32-pin
PLCC or 44-pin PLCC package. Operating temperature
range, maximum current limit, maximum battery volt-
age, minimum dc loop, and protection resistor values
will influence the overall thermal performance. This
section shows the relevant design equations and con-
siderations in evaluating the SLIC thermal perfor-
mance.
First, consider the L8551 SLIC in a 44-pin PLCC pack-
age. The still air thermal resistance is 47
°
C/W; how-
ever, this number implies zero airflow as if the L8551
were totally enclosed in a box. A more realistic number
would be 43
°
C/W. This is an experimental number that
represents a thermal impedance with no forced airflow
(i.e., from a muffin fan), but from the natural airflow as
seen in a typical switch cabinet.
The SLIC will enter the thermal shutdown state at typi-
cally 165
°
C. The thermal shutdown design should
ensure that the SLIC temperature does not reach
165
°
C under normal operating conditions.
Assume a maximum ambient operating temperature of
85
°
C, a maximum current limit of 45 mA, and a maxi-
mum battery of –52 V. Further, assume a (worst case)
minimum dc loop of 100
and that 100
protection
resistors are used at both Tip and Ring.
1. T
TSD
– T
AMBIENT(max)
= allowed thermal rise.
165
°
C – 85
°
C = 80
°
C
2. Allowed thermal rise = package thermal
impedance
SLIC power dissipation.
80
°
C = 43
°
C/W
SLIC power dissipation
SLIC power dissipation (P
DISS
) = 1.9 W
Thus, if the total power dissipated in the SLIC is less
than 1.9 W, it will not enter the thermal shutdown state.
Total SLIC power is calculated as:
Total P
DISS
= Maximum battery
Maximum
current limit + SLIC quiescent power.
For the L8551, SLIC quiescent power (P
Q
) is approxi-
mated at 0.135 W. Thus,
Total P
DISS
= (–52 V
45 mA) + 0.135 W
Total P
DISS
= 2.34 W + 0.135 W
Total P
DISS
= 2.475 W
The power dissipated in the SLIC is the total power dis-
sipation less the power that is dissipated in the loop.
SLIC P
DISS
= Total power – Loop power
Loop power = (I
LIM
)
2
(R
DCLOOP
min + 2R
P
)
Loop power = (45 mA)
2
(100
+ 200
)
Loop power = 0.61 W
SLIC power = 2.475 W – 0.61 W
SLIC power = 1.865 W < 1.9 W
Thus, in this example, the thermal design ensures that
the SLIC will not enter the thermal shutdown state.
The next example uses the 32-pin PLCC package and
demonstrates the technique used to determine the
maximum allowed current.
In this example, assume a 0
°
C to 70
°
C operating
range. Thus,
T
TSD
– T
AMBIENT (max)
= Allowed thermal rise
165
°
C – 70
°
C = 95
°
C
To estimate the open-air thermal impedance, use the
43
°
C/W parameter from the 44-pin PLCC and ratio the
lead count.
Again;
Allowed thermal rise = Thermal impedance
SLIC
power dissipation
95
°
C = 59
°
C/W
SLIC power dissipation
SLIC P
DISS
= 1.6 W
In this example again assume the dc loop + 2
protec-
tion resistors = 300
, then;
(I
LIM
BAT
max) + P
Q
– (I
LIM
)
2
(R
DC
+ 2 R
P
) = 1.7 W
I
52 + 0.135 – I
2
300 = 1.7 W
300 I
2
– 52 I + 1.565 = 0
Thermal Impedance (32-PLCC)
43
°
C/W
32
59
°
C/W
=
+
=
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