PBL 385 73
Few hints how to widen
some of the specification
limits. See figure 20.
V
16
14
12
10
8
V telephone line
V line
+
How to increase the maximum allowable
line current ? The current capability can
be extended by using more sophisticated
cooling methods but can also be done as
follows:
RA
V pin 1
W
RA
1
PBL 38573
V pin 2
Wpackage
The maximum line current capability is
mainly set by the power rating of the
package used for the speech circuit
chip.The power used in the speech circuit
loop is divided between the IC and the
resistor R6. When a resistor RA is added
into the loop it will drain power at high line
currents from the IC but observe that it will
also limit the receiver and transmitter sig-
nal swing on the line.These have to be
recalculated at introduction of a resistor RA.
Also observe that at high line currents the
R6 has to withstand close to 1 watt.
In case a higher DC - mask is desired, it
can be increased by connecting diodes in
series with the pin 4. Each diode will rise
the level by ≈ 0.7V. The diode in opposite
direction will preserve the function of the
fast start circuit.
2
11
6
4
WR6
R6
2
I
L
mA
20
40
60
80
100
120
The rest like in figure 4.
Figure 20. Power distribution in the speech circuit using RA.
Z
line
1
13
14
PBL 385 73
9
AT
AM
AR
+
10
50µA
DC-supply
3
2
+
4
12
11
7
6
5
8
1k
V
+
How to determin the chip
temperature at various line
currents. See figures 21 a and b.
75Ω
10kΩ
47µF
R6
R6 = 10k to calibrate the temperature characteristic of the diode at ≈ 0 line current.
Figure 21a. Chip temperature measuring circuit.
It is important to know the chip tempera-
ture when using the component at high line
currents but especially when using the SO-
package. Obviously the mounting of the
component on the PC-board has a great
influence on the heat transfer from the chip
to the ambient. A couple rules of thumb
are; leave as much copper foil under and
around the package as possible. Use, if
possible, temperature conducting glue at
assembly of the package. Do not mount
the R6 close to the circuit and sometimes
it´s good to split the R6 into several
components in order to avoid heat con-
centration.
mV
690
SO - package
K=2mV/°C
A simple methode to ensure that the chip
temperature is within safe limits at the line
current in question is to measure the the
voltage at pin 10 to ground with pin 9
grounded. This measured voltage being
over 0.5 volts indicates a safe chip
temperature but going under this value
some additional calculations are to be
made to determine the real chip tempera-
ture.
500
°C
120
25
Figure 21b. Chip temperature expressed as a voltage across a diode on the chip
between pins 9 and 10.
12
ERICSSON [ ERICSSON ]
