Data Sheet
March 1997
L7554 Low-Power SLIC
This is an equivalent circuit for adjusting both the dc
feed resistance and overhead voltage together.
Applications (continued)
dc Applications (continued)
Adjusting dc Feed Resistance
The adjustments can be made by the simple superposi-
tion of the overhead and dc feed equations:
R1 ||
R2 + R1 ||
||
25 kΩ R3
VOH = 6.5 + 20
---------------------------------------------
||
25 kΩ R3
The dc feed resistance may be adjusted with the help of
Figure 28.
R1 ||
R3 + R1 ||
25 kΩ
Rdc = 113 Ω +500 Ω
-----------------------------------
25 kΩ
R1
25 kΩ ± 30%
When selecting external components, select R1 on the
order of 5 kΩ to minimize the programming inaccuracy
caused by the internal 25 kΩ resistor. Lower values can
be used; the only disadvantage is the power consump-
tion of the external resistors.
DCR
R3
Loop Range
DCOUT
The equation below can be rearranged to provide the
loop range for a required loop current:
12-2560 (C)
Figure 28. Equivalent Circuit for Adjusting the dc
Feed Resistance
VBAT – VOH
RL =
– 2RP – Rdc
----------------------------
IL
Off-Hook Detection
∆VDCR
Rdc = 113 Ω + 500 Ω
= 113 Ω +500 Ω
--------------------
∆VDCOUT
The loop closure comparator has built-in longitudinal
rejection, eliminating the need for an external 60 Hz fil-
ter. This applies in both powerup and low-power scan
states. The loop-closure detection threshold is set by
resistor RLCTH. Referring to Figure 30, NLC is high in an
on-hook condition (ITR = 0, VDCOUT = 0), and
VLCTH = 0.05 mA xRLCTH. The off-hook comparator
goes low when VLCTH crosses zero and then goes neg-
ative:
R1 ||
R3 + R1 ||25 kΩ
25kΩ
---------------------------------
Adjusting Overhead Voltage and dc Feed
Resistance Simultaneously
VLCTH = 0.05 mA x RLCTH + VDCOUT
= 0.05 x RLCTH – 0.125 V/mA x ITR
RLCTH(kΩ) = 2.5 x ITR(mA)
The following paragraphs describe the independent set-
ting of the overhead voltage and the dc feed resistance.
If both need to be set to customized values, combine
the two circuits as shown in Figure 29.
RP
PT
ITR
R1
–0.125 V/mA
DCOUT
+
–
25 kΩ ± 30%
RL
RLCTH
DCR
PR
R3
R2
RP
LCTH
NLC
+
–
0.05 mA
–5 V
DCOUT
12-2561 (C)
12-2553.a (C)
Figure 29. Adjusting Both Overhead Voltage and dc
Feed Resistance
Figure 30. Off-Hook Detection Circuit
Lucent Technologies Inc.
22
AGERE [ AGERE SYSTEMS ]
