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參數(shù)資料
| 型號： | MC33121P |
| 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, PDIP20 |
| 封裝： | PLASTIC, DIP-20 |
| 文件頁數(shù)： | 13/32頁 |
| 文件大小： | 570K |
| 代理商： | MC33121P |
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MC33121
MOTOROLA
13
Figure 27 is representative of the DC loop current path
(bold lines). On a long line (RL > 400
), the loop current can
be determined from the following equation:
IL
VEE– 3.6 V
13
RRF
{(RL
5)
13}
(1)
On short lines (RL < 400
), the three diodes across the
12.4 k resistor clamp the voltage at RFO, thereby preventing
the RXI current from increasing as the load resistance is
decreased. The maximum loop current is:
IL(max)
1.85 V
102
RRF
(TA
25
°
C)
(2)
Due to the temperature dependence of a diode’s forward
voltage, the maximum loop current will change with temper-
ature by
– 0.3%/
°
C.
The battery feed resistance (
VTIP/
IL) is
depends on the loop current, VEE, RRF, and is a valid param-
eter only on long lines where the current limit is not in effect.
On short lines, the feed resistance is high since the loop cur-
rent is clamped at a near constant level. The AC impedance
(Return Loss) however, is not determined nor affected by the
DC parameters. See the Applications Section for Return
Loss information.
400
, but
Transmit Path
The transmit path, shown in Figure 28, consists of an inter-
nal amplifier which has inputs at CP and CN, and its output at
TXO. The gain is internally fixed at 0.328 V/V (– 9.7 dB). The
output is in phase with the signal at CP (normally the same
as TIP), and is out of phase with the signal at CN. The signal
at TXO is also out of phase with that at VRX, the receive sig-
nal input, described in another section.
The TXO output can swing
current capability of
±
800
μ
A peak (
±
275
μ
A minimum). The
load on TXO is the parallel combination of RTX1 and the
RRO network (described later). TXO is nominally internally
biased at the VAG DC level, but has an offset which varies
with loop current.
In normal applications, the signal at CP/CN is reduced
slightly from that at Tip/Ring by the voltage divider composed
of the external RC resistors, and the internal 31 k resistors.
The value of the RC resistors depends on the transient
3.0 Vp–p, with a nominal
protection needed, described in another section, with 1.0 k
resistors being suitable for most applications. The resulting
signal at TXO needs to be gained up to obtain 0 dB from Tip/
Ring to VTX (the 4–wire output). The common method
involves an external op amp, as shown in Figure 28, with a
gain of RTX2/RTX1. The gain from VL to VTX is:
VTX
VL
RTX2
31 k
0.328
RTX1
(RC
31 k)
(3)
If a codec/filter is used, many of which include an internal
op amp, a separate op amp is not needed. CTX is primarily
for DC blocking (of the TXO offset), and is usually large
(1.0
μ
F) so as to not affect the gain.
Receive Path
The receive path, shown in Figure 29, consists of the input
at RXI, the transistor driver amplifiers, the external transis-
tors, and the load at Tip/Ring.
RXI is a virtual ground (DC level = VAG) and is a current
input. Current flow is out of the pin. The RXI current is mir-
rored to the two transistor drivers which provide a gain of
102. the two external transistors are then two current
sources, in series, operating at the same value. An additional
internal circuit (not shown) balances the two current sources
to maintain operation in their linear region.
The load current (through RL) is slightly different from the
transistor current due to the sense resistors RC and RS. The
sense resistors add to the DC loop current, but subtract from
the AC load current.
In normal operation, the current at RXI is composed of a
DC current (from RFO), an AC current (from VRX) which is
the receive signal, and an AC current from TXO, which is the
feedback signal to set the return loss (setting the return loss
is discussed in the section on AC Terminating Impedance).
The resulting AC signal at Tip is inverted from that at VRX,
while the signal at Ring is in phase with VRX.
The resistors RP are for transient protection, and their
value (defined in another section) depends on the amount of
protection required. A nominal value of 100
is suitable for
most applications.
The system receive gain, from VRX to Tip/Ring, is not
described in this section since in normal applications, it
involves the feedback which sets the AC terminating imped-
ance. The Applications Section discusses these in detail.
Figure 28. Transmit Path
–
+
–
+
TIP
RING
1.0 k
RC
CP 17
31 k
AC
VDD
11
AV = 0.328
VQB
VAG
RTX1
CTX
RTX2
VTX
NOTE: Op amp may be part
of a codec/filter.
VL
1.0 k
RC
CN 4
31 k
AC
TXO
9
MC33121
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