LUCL9218AAR-D AGERE [Agere Systems], LUCL9218AAR-D Datasheet - Page 27

• Manufacturer Part Number: LUCL9218AAR-D
• Description: Low-Cost Line Interface
• Manufacturer: AGERE [Agere Systems]

Data Sheet
November 2001
Applications
Power Derating
Operating temperature range, maximum current limit,
maximum battery voltage, minimum dc loop, and pro-
tection resistor values will influence the overall thermal
performance. This section shows the relevant design
equations and considerations in evaluating the SLIC
thermal performance.
Consider the L9218 SLIC in a 28-pin PLCC package.
The still-air thermal resistance on a 2-layer board is
typically 43 °C/W.
The SLIC will enter the thermal shutdown state at a
minimum of 150 °C. The thermal shutdown design
should ensure that the SLIC temperature does not
reach 150 C under normal operating conditions.
Assume a maximum ambient operating temperature of
85 °C, a maximum current limit of 25 mA (including tol-
erance), and a maximum battery of –52 V. Further-
more, assume a (worst-case) minimum dc loop of
200 , and that 50
both tip and ring.
1. T
2. Allowed thermal rise = package thermal
Thus, if the total power dissipated in the SLIC is less
than 1.51 W, it will not enter the thermal shutdown
state. Total SLIC power is calculated as:
For the L9218, SLIC quiescent power (P
at 0.158 W. Thus,
The power dissipated in the SLIC is the total power dis-
sipation minus the power that is dissipated in the loop.
Agere Systems Inc.
impedance
65 °C = 43 °C/W
SLIC power dissipation (P
Total P
current limit (including effects of accuracy)
+ SLIC quiescent power
Total P
Total P
Total P
SLIC P
Loop power = (I
150 °C – 85 °C = 65 °C
TSD
– T
DISS
DISS
DISS
DISS
DISS
AMBIENT(max)
= maximum battery
= (–52 V
= 1.365 W + 0.158 W
= 1.523 W
= total power – loop power
•
SLIC power dissipation.
(continued)
LIM
•
protection resistors are used at
)
SLIC power dissipation
2
= allowed thermal rise.
•
•
[25 mA
(R
dcLOOP
DISS
) = 1.51 W
•
•
min + 2R
1.05]) + 0.158 W
maximum
Q
) is maximum
P
)
Thus, in this example, the thermal design ensures that
the SLIC will not enter the thermal shutdown state.
Pin-for-Pin Compatibility with L9217/L9219
The L9218 is an exact pin-for-pin replacement for the
L9217/19. The one minor exception is L9217/19 has
three logic control inputs: B0, B1, and B2. The L9218
has only two logic control inputs, B0 and B1. B2 in the
L9217/19 is pin 13. Pin 13 in L9218 is NC, so a con-
nection between the controller and pin 13 will not affect
L9218 operation. This allows an exact footprint match
with L9217/19.
PCB Layout Information
Make the leads to BGND and V
for thermal and electrical reasons. Also, maximize the
amount of PCB copper in the area of (and specifically
on) the leads connected to this device for the lowest
operating temperature.
When powering the device, make certain that no exter-
nal potential creates a voltage on any pin of the device
that exceeds the device ratings. In this application,
some of the conditions that cause such potentials dur-
ing powerup are the following:
1. An inductor connected to PT and PR (this can force
2. Inductance in the V
This device is normally used on a circuit card that is
subjected to hot plug-in, meaning the card is plugged
into a biased backplane connector. In order to prevent
damage to the IC, all ground connections must be
applied before, and removed after, all other connec-
tions.
an overvoltage on V
devices if the V
the V
voltage).
Loop power = (25 mA
(200
Loop power = 0.207 W
SLIC power = 1.523 W – 0.207 W = 1.28
SLIC power = 1.28 W < 1.51 W
BAT
+ 100
L9218A/G Low-Cost Line Interface
filter capacitor to cause a destructive over-
BAT
)
connection chatters).
BAT
BAT
lead (this could resonate with
•
through the protection
1.05)
BAT
2
•
as wide as possible
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