MC33121 Datasheet, PDF (23/32 Pages) Motorola, Inc – LOW VOLTAGE SUBSCRIBER LOOP INTERFACE CIRCUIT

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MC33121
STILL AIR, SOLDERED
60
TO A G–10 PC BOARD
50
40
30
25
35
45
55
65
75
85
TA, °C
Figure 39. Thermal Resistance
(Junction–to–Ambient)
Selecting the Transistors
The specifications for the two loop current pass transistors
involve their current gain, voltage rating, and power dissipa-
tion capabilities at the highest ambient temperatures. Power
dissipation during both normal operation and faults must be
considered when determining worst case situations. Gener-
ally, more power is dissipated during a fault condition than
during normal operation.
The transistors’ minimum beta is recommended to be 40 at
the loop currents involved in the application. A lower beta
could degrade gain and balance performance. Maximum
beta should be less than 500 to prevent possible oscillations.
Darlington type transistors should not be used. The voltage
rating should be consistent with the maximum VEE, expected
transients, and the protection scheme used.
Referring to Figure 27, during normal operation the loop
current and the voltage across the transistors are both at a
maximum when the load impedance (RL) is at a minimum.
The loop current is determined by RRF and the graphs of
Figures 5 – 7. The voltage across each transistor is deter-
+ ^{Ť Ť ) ) mined from the following:}
VT
VEE – 2.1 – [(65 2RP
2
RL) • IL]
(22)
The power in each transistor is then (VT • IL). The voltage
across the two transistors will always be nearly equal during
normal operation, resulting in equal power dissipation. The
graph of Figure 24 indicates the power dissipated in each
transistor where RP = 100 Ω .
During a fault condition, depicted in Figure 38, if the leak-
age resistance from Tip to VEE or from Ring to VCC is less
than that shown in Figures 12 – 14 (when off–hook), the
MC33121 will power down the transistors to protect them
from overheating. Should the leakage resistance be slightly
higher than that shown in the graphs, however, and the fault
detection has not been activated, the power in one transistor
(in a single fault, both transistors in a double fault) will be
higher than normal. The power will depend on VEE, RL, RP
and the leakage resistance. Table 3 is a guide of the power in
the transistor dissipating the higher power level.
The power (in watts) in the two right columns indicates the
power dissipated by that transistor if it is carrying the maxi-
mum fault current. The system designer should attempt to
predict possible fault conditions for the system, and then
MOTOROLA
measure the conditions on the transistors during the worse
case fault(s).
Table 3. Transistor Power During a Fault
VEE
RL
PPNP
PNPN
– 42
150
– 24
150
– 42
600
– 24
600
0.835
0.257
0.601
0.109
0.615
0.176
0.185
0.057
For most applications involving a maximum loop current of
30 – 40 mA, and a maximum TA of + 85°C, and where faults
may occur, the MJD243 and MJD253 DPAK transistors are
recommended. When mounted as described in their data
sheet, they will handle both the normal loop current as well
as most fault conditions. If faults are not expected to occur in
a particular application, then smaller package transistors,
such as MPS6717 and MPS6729, may be used. Each ap-
plication must be evaluated individually when selecting the
transistors.
Other possible transistors which can be considered:
PNP
MJD253–1
MJE253
MJD32
MJD42
MJD350
TIP30A,B,C
NPN
MJD243–1
MJE243
MJD31
MJD41
MJD340
TIP29A,B,C
Longitudinal Current Capability
The maximum longitudinal current which can be handled
without distortion is a function of loop current, battery feed re-
sistance, the longitudinal impedance, and the components
on ST2.
Since the pass transistors cannot pass current in the re-
verse direction, the DC loop current provides one upper
boundary for the peak longitudinal current plus peak speech
signal current. The battery feed resistance determines, in ef-
fect, the DC voltage across the transistors, which is a mea-
sure of the headroom available for the circuit to handle the
peak longitudinal voltage plus peak speech signal voltage.
The longitudinal impedance, determined by the RS resistors
(equation 4), determines the longitudinal current for a given
longitudinal voltage.
While analysis of the above items may yield one value of
maximum longitudinal current, a different limit (which may
be higher or lower) is imposed by the capacitor CT, and any
pulldown resistance RT, on Pin 12 (ST2). This is due to the
fact that the sense currents at TSI and RSI will be alternately
mismatched as Tip and Ring move up and down together in
the presence of longitudinal signals. When the longitudinals
are strong, the internal fault detect circuit is activated with
each 1/2 cycle, which attempts to switch ST2 low (see the
section on Fault Detection). The speed at which ST2 can
switch low is a function of both the external capacitor, CT and
any pulldown resistance, RT.
The graphs of Figures 25 and 26 indicate the maximum
longitudinal current which can be handled (in Tip and in Ring)
without distortion or causing ST2 to switch low.
PC Board Layout Considerations
PC board considerations include thermal, RFI/EMI, tran-
sient conditions, interconnection of the four wire side to the
MC33121
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