Datasheet

DS2148/DS21Q48
51 of 73
Figure 7-3. Protected Interface Using External Receive Termination
NOTES:
1) All resistor values are ±1%.
2) C1 = 0.1µF.
3) S is a 6V transient suppresser.
4) D1 to D4 are Schottky diodes.
5) The fuses are optional to prevent AC power line crosses from compromising the transformers.
6) Rp resistors exist to keep the Fuses from opening during a surge. If they are used, then Rr must be adjusted to match the line
impedance.
7) Rr = 37.5Ω for 75Ω, 60Ω for 120Ω E1 systems, or 50Ω for 100Ω T1 lines.
8) The Rt resistors are used to increase the transmitter return loss (Table 7-1). No return loss is required for T1 applications.
9) The transmit transformer turns ratio (N) would be 1:1.36 for 5V operation.
10) The 68µF is used to keep the local power plane potential within tolerance during a surge.
RTIP
RRING
TTIP
TRING
Receive
Line
N:1
(larger winding
toward the network)
DS2148
0.47µF
(non-
polarized)
V
DD
(21)
V
SS
(22)
V
DD
(36)
V
SS
(35)
2.048MHz
(this clock can also
be 1.544MHz for T1
only applications)
MCLK
+V
DD
S
C1
D1
D2
D3 D4
1:1
0.1µF
470
470
Rp
Fuse
Rp
Fuse
R
r
R
r
Rt
Rt
(optional)
Transmit
Line
Rp
Fuse
Rp
Fuse
(optional)
0.1µF
0.1µF
+V
DD
0.01µF
68µF
10µF
10µF