Data Sheet
May 2001
L8576B Dual Ringing SLIC
Next, calculate the power dissipated in the SLIC:
Applications (continued)
ac Design (continued)
Components of the SLIC power dissipation are quies-
cent power of VCC and VBAT and loop current associ-
ated with VBAT and Aux Bat. These can be calculated
as follows:
Typically, values of 0.1 µF to 0.47 µF capacitors are
used for dc blocking. The addition of blocking capaci-
tors will cause a shift in the return loss and hybrid bal-
ance frequency response toward higher frequencies,
degrading the lower-frequency response. The lower
the value of the blocking capacitor, the more pro-
nounced the effect is, but the cost of the capacitor is
lower. It may be necessary to scale resistor values
higher to compensate for the low-frequency response.
This effect is best evaluated via simulation. A PSPICE*
model for the L8576B is available.
WVCC(quiescent) = VCC x ICC(Max)(quiescent) x 2 channels.
WVBAT(quiescent) = |VBAT(Max)| x IBAT(Max)(quiescent) x 2 chan-
nels.
WVBAT(loop current) = (|VBAT(Max)| – 4 V) x 3 mA x 2 chan-
nels.
WAux Bat(loop current) = (|Aux Bat(Max)| – 4 V) x (ILIM – 3 mA)
x 2 channels.
Where:
4 V is the minimum overhead voltage, and 3 mA is
VBAT’s contribution to loop current.
Design equation calculations seldom yield standard
component values. Conversion from the calculated
value to standard value may have an effect on the ac
parameters. This effect should be evaluated and opti-
mized via simulation.
For example, substituting values from the data sheet:
VCC = 5 V
ICC(Max)(quiescent) = 5.5 mA
VBAT(Max) = –70 V
IBAT(Max)(quiescent) = 4 mA
Aux Bat(Max) = –23.3 V
ILIM = 24 mA
Use of an Auxiliary Battery Supply
A second lower-voltage battery supply can be used
with the L8576B in order to lower the overall power
consumption on a short-loop design. For long loops,
any power savings will be negated, since long loops
are supplied by the main battery voltage. The auxiliary
battery would be connected to pins 9 and 37 in lieu of
the RPWR resistors. When the external RPWR resistors
are removed, more power will be dissipated in the SLIC
so internal SLIC power dissipation must be examined.
The following powers are calculated:
WVCC(quiescent) = 5 x 0.0055 x 2 = 0.055
WVBAT(quiescent) = |–70| x 0.004 x 2 = 0.56
WVBAT(loop current) = (|–70| – 4) x 0.003 x 2 = 0.396
WAux Bat(loop current) = (|–23.3| – 4) x (0.024 – 0.003) x 2 =
0.8106
The sum of the four powers is 1.822 W.
Finally, calculate the maximum ambient tempera-
ture allowed for the calculated power dissipation:
First, determine the auxiliary battery voltage:
The auxiliary battery should be set 8 V greater than the
maximum tip/ring loop voltage on the longest allowed
loop, when both channels are off-hook and in current
limit.
TA(max) = Tj – (RθJA x PDISS SLIC(max))
The L8576’s 44-pin PLCC exhibits a 43 °C/W thermal
resistance if in an enclosure with natural airflow. The
maximum operating temperature of the SLIC is 150 °C.
Thermal shutdown occurs typically at 160 °C.
Aux Bat(MAX) = [(ILIM x RLOOP) + VOH] TOLVBAT
Where:
For example:
ILIM = dc current limit set by RPROG (usually 0.024).
RLOOP = maximum loop resistance supported (tele-
phone plus line resistance plus protection
resistors).
TA(max) = 150 – (43 x 1.822)
= 150 – 78.4
= 71.7 °C
VOH = overhead voltage.
TOLVBAT = battery tolerance, for a battery tolerance of
±5%, use 1.1.
The above scenario would allow operation up to 70 °C.
For example, using the recommended 24 mA current
limit, an overhead voltage of 8 V, and a maximum loop
length of 550 Ω, the maximum auxiliary battery voltage
is 23.3 V.
* PSPICE is a registered trademark of Cadence Design Systems, Inc.
24
Agere Systems Inc.
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