參數(shù)資料
型號(hào)： MC33121FN
廠商： MOTOROLA INC
元件分類： 模擬傳輸電路
英文描述： LOW VOLTAGE SUBSCRIBER LOOP INTERFACE CIRCUIT
中文描述： TELECOM-SLIC, PQCC28
封裝： PLASTIC, LCC-28
文件頁(yè)數(shù)： 16/32頁(yè)
文件大?。?/td> 570K
代理商： MC33121FN
MC33121
16
MOTOROLA
current at an ambient temperature of + 25
°
C, and after the IC
has reached thermal equilibrium (approx. 10 minutes).
Although the maximum loop current is primarily a function
of the RRF resistor, it is also affected by ambient tempera-
ture, and slightly by VEE. The ambient temperature effects
are due to the temperature dependence of the diodes’ for-
ward voltage drop, causing the maximum loop current to
change by
– 0.3%/
°
C. Changing VEE affects the maximum
current in that the power dissipation is changed, thereby
changing the die temperature, which affects the diodes’ volt-
age.
The maximum loop current is affected slightly (< 5%) by
the choice of the RS and RC resistors, since the sense cur-
rents through those resistors add to the current supplied by
the transistors.
The battery feed resistance is determined by RRF, and is
not adjustable independently of the current limit. Defined as
VTIP/
IL, it is
400
, and is a valid parameter only on long
lines where the current limit is not in effect. On short lines, the
feed resistance is high since the loop current is clamped at a
near constant level. The AC impedance (Return Loss) how-
ever, is not determined nor affected by these DC parameters.
Return loss is discussed in another section.
If the application requires that the current limit value have
a low temperature dependence, refer to the section following
this design sequence which describes an alternate configu-
ration.
2) Main Protection Resistors (RP) and Transient
Currents
The purpose of the protection resistors (RP), along with
the 4 clamp diodes shown in Figure 4, is to absorb the bulk of
the transient energy when transient voltages come in from
the phone line. The resistor value must be selected to limit
the transient current to a value which can be tolerated by the
diodes, while dissipating the energy. The recommended
value shown (100
) will limit the current from a 1500 V tran-
sient to 15 A, which can be carried by 1N4002 diodes under
surge conditions. The resistors must be of a type which can
tolerate the high instantaneous energy associated with tran-
sients. Resistor manufacturers should be consulted for this
information.
Referring to Figure 4, a positive transient on either Tip or
Ring, or both, will cause the transient current to be delivered
to Ground. A negative transient will cause the transient cur-
rent to come from the VEE supply line. Therefore, the PC
board track supplying VCC and VEE to the MC33121 must
be designed to carry the transient currents as well as the nor-
mal operating currents. Additionally, since a negative tran-
sient will cause a current flow
out
of the power supply’s
negative output, which is opposite to the normal flow of cur-
rent, provisions must be made for this reverse current flow.
One suggested method is to place a zener transient suppres-
sor (1N6287 for – 42 V, 1N6282 for – 28 V and – 24 V) across
the battery supply pins (VCC to VEE) physically adjacent to
the MC33121. The inductance associated with PC board
tracks and wiring will result in insufficient protection for the
MC33121 if the suppressor is located at the opposite end of
the line card, or at the power supplies.
Transient currents can be reduced by increasing the value
of RP, with an upper limit determined by the DC conditions on
the longest line (highest loop resistance) and minimum VEE
supply voltage. These conditions determine the minimum DC
voltage across the transistors, which must be sufficient to
handle the largest AC (transmit and receive) signals. If too
large a value is selected for RP, the AC signals will be
clipped. It is recommended that each transistor have no less
than one volt (DC) across their collector to emitter. System
AC specifications may require more than this.
Since the RP resistors are within the loop, their tolerance
can be
±
5% with no substantial degradation of longitudinal
balance. A
±
10% tolerance (20% mismatch) will degrade
balance by
4.0 dB on a 65 dB device.
Figure 31.
Figure 32. RC Protection Resistors
–
+
TIP
RING
RC
1.0 k
CP
VCC
VEE
31 k
AC
VDD
TXO
AV =
0.328
MC33121
RC
1.0 k
CN
VCC
VEE
31 k
AC
VQB
3) Selecting the RC Resistors
The primary purpose of the RC resistors is to protect the
CP and CN pins from transient voltages and destructive cur-
rents. Internally, these pins have clamp diodes to VCC and
VEE rated for a maximum of 1.0 A under surge conditions
only (Figure 32). The 1.0 k
resistors shown in the figures,
for example, will provide protection against surges up to
1.0 kV. Resistor manufacturers must be consulted for the
proper type of resistor for this environment.
The RC resistors are in series with internal 31 k
resistors,
and therefore form a voltage divider to the inputs of the trans-
mit amplifier, as shown in Figure 32. This will affect the trans-
mit gain, receive gain, return loss, and transhybrid rejection
(described in subsequent sections). The tolerance of the RC
resistors depends on the value selected for them, since any
mismatch between them will create a differential voltage at
CP and CN when longitudinal voltages are present on Tip
and Ring. To ensure a minimum of 58 dB of longitudinal bal-
ance, the resistors’ absolute value must not differ by more
than 39
. With a nominal value of 1.0 k
, their tolerance
must be
±
2%, or less. If their nominal value is 390
or less,
their tolerance can be
±
5%.
4) Longitudinal Impedance (ZLong) — Selecting the
RS Resistors
The longitudinal impedance is determined by the RS re-
sistors at the TSI and RSI pins according to the following
equation:
RS
ZLong
100
51
(4)
ZLong is defined as VLong/ILong as shown in Figure 33;
for RS = 9.1 k
, ZLong = 180
. The calculated value
of ZLong includes the fact that the RS resistors are in parallel
with the synthesized impedance. The tolerance of the RS
resistors therefore depends on how much mismatch can be
tolerated between the longitudinal impedances at Tip and at
Ring. Calculations indicate the two RS resistors can have a
±
5% tolerance, and still comfortably provide a minimum of
58 dB longitudinal balance.
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