L9215A/G
Short-Loop Sine Wave Ringing SLIC
Data Sheet
September 2001
Overhead Voltage
Applications (continued)
Active Mode
dc Loop Current Limit (continued)
Overhead is programmable in the active mode via an
applied voltage source at the device’s OVH control
input. The voltage source may be an external voltage
source or derived via a resistor divider network from
the VREF SLIC output or an external voltage source. A
programmable external voltage source may be used to
provide software control of the overhead voltage. The
overhead voltage (VOH) is related to the OVH voltage
by:
Note that the overall current-limit accuracy achieved
will not only be affected by the specified accuracy of
the internal SLIC current-limit circuit (accuracy associ-
ated with the 67 term), but also by the accuracy of the
voltage source and the accuracy of any external resis-
tor divider network used and voltage offsets due to the
specified input bias current. Tolerance of the current
limit is ±8%. If a resistor divider from VREF is used, it is
recommended that the sum of the two resistors be
greater than 100 kΩ.
VOH = 6.0 V + 5 * VOVH (V)
The above equations describe the active mode steady-
state current-limit response. There will be a transient
response of the current-limit circuit upon an on- to off-
hook transition. Typical active mode transient current-
limit response is given in Table 19.
Overall accuracy is determined by the accuracy of the
voltage source and the accuracy of any external resis-
tor divider network used and voltage offsets due to the
specified input bias current. If a resistor divider from
VREF is used, a lower magnitude resistor will give a
more accurate result due to a lower offset associated
with the input bias current; however, lower value resis-
tors will also draw more power from VREF.
Table 19. Typical Active Mode On- to Off-Hook Tip/
Ring Current-Limit Transient Response
Parameter
dc Loop Current:
Value
Unit
Note that a default overhead voltage of 6.0 V is
achieved by shorting input pin OVH to analog ground.
ILIM + 60
mA
Active Mode
RLOOP = 100 Ω On- to Off-hook
Transition t < 5 ms
The default overhead provides sufficient headroom for
an on-hook transmission of a 3.14 dBm signal into
900 Ω.
dc Loop Current:
Active Mode
RLOOP = 100 Ω On- to Off-hook
Transition t < 50 ms
ILIM + 20
mA
mA
Overhead voltage may need to be increased to accom-
modate on-hook transmission of higher-voltage sig-
nals, such as meter pulse. The following example is
meant to illustrate the design procedure that can be fol-
lowed.
dc Loop Current:
Active Mode
ILIM
Assume we need on-hook transmission of a 1.0 Vrms
meter pulse into 200 Ω. Further, assume 50 Ω protec-
tion resistors are used.
RLOOP = 100 Ω On- to Off-hook
Transition t < 300 ms
VOH = 6.0 V + (1+ [2 * Rp]/200) * Vpeak
VOH = 6.0 + (1+ [2 * 50]/200) * 1 (1.414)
VOH = 8.121 V
Agere Systems Inc.
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