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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)： | 26/32頁 |
| 文件大小： | 570K |
| 代理商： | MC33121P |
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MC33121
26
MOTOROLA
Alternate Circuit Configurations
a) Loop Current Limit
Replacing the RRF resistor with the circuit in Figure 43 will
change the DC loop current characteristics in two ways from
the graphs of Figures 5 – 7; a) the maximum loop current on
a short line can be reduced while increasing the current on a
long line, and b) the temperature dependence of the maxi-
mum current is reduced to the TC of the external reference
diode.
Figure 43. Alternate Current Limit Circuit
VAG
RXI
RRF1
LM385–1.2
RFO
RRF2
The LM385–1.2 is a precision temperature stable zener
diode. As the load impedance at Tip and Ring is reduced, the
voltage at RFO goes increasingly negative. When the zener
diode is turned on, the current into RXI is then clamped at a
value determined by RRF1 and the zener diode. To calculate
the two resistors, use the following procedure:
RRF1 must be > 0.7
(RRF1 + RRF2);
Determine RRF1 to set the current limit on a short line by
using the following equation:
102
1.23 V
IL(max)– 3.0 mA
RRF1
(23)
Then using Equation 1 calculate RRF for the long line cur-
rent. RRF2 is then determined by;
RRF2 = RRF – RRF1
(24)
Figure 44 illustrates one example using the above circuit.
Comparing this graph to the 5100
curve of Figure 7 shows
a substantial decrease in the current limit (at RL = 0), result-
ing in reduced power consumption and dissipation. Use of
this circuit does not affect the hookswitch or fault thresholds.
b) Protection Scheme
The protection circuit shown in Figure 45 has the advan-
tage of drawing
90% of the transient current from ground
(VCC) on a negative transient, rather than from the VEE line
as the circuit of Figure 4 does. The majority of the transient
current flows through the RP resistors and the Mosorbs while
a small amount (
and the CP, CN, RSI pins. On a positive transient, all the cur-
rent is directed to ground. The diode in the NPN’s collector
prevents reverse current through the base–collector junction
of the transistor during a negative transient.
10%) flows through the sense resistors
Figure 44. Loop Current versus Loop Resistance
Alternate Loop Current Limit Configuration
50
0
40
30
20
10
200
400
600
800
I
RL, LOOP RESISTANCE (
)
RRF1 = 4.3 k
RRF2 = 820
RS = 9.1 k
VEE = –24 V
TA = 25
°
C
Figure 45. Alternate Protection Scheme
VCC
EP
BP
CP
TSI
RSI
CN
BN
EN
VEE
14 V
MC33121
All zener diodes are 7.0 V except as noted.
14 V
1.5KE15
0.01
100 V
RP
TIP
RING
1.0 k
9100
1N4002
–24 V
0.01
100 V
RP
9100
1.0 k
1.5KE30
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