ZLR964222L
Line Module User Guide
14
Microsemi Corporation Confidential and Proprietary
Overcurrent protection is provided by the
Bourns MF-SD013/250
dual PTC devices (or equivalent) which ramp to a
high impedance value once the threshold current has been exceeded. Below this threshold current, the devices act
as nominal 5
resistors. The hold current at 23
C is 0.13 A. The
MF-SD013/250
is a dual PTC that uses a polymer
technology. Other PTC options are available such as individual polymer and ceramic single or dual PTCs with
nominal resistance of up to 50
. Be aware that the effective resistance of ceramic PTCs is considerably less in the
presence of fast-edged transients. Also, note that higher values PTCs will limit the maximum available ringing
voltage. AC coefficients should take the PTC resistances into account or performance may not be as expected.
Microsemi has standard coefficients available for various PTC values.
The protection solution provided by the Microsemi reference design is suitable for environments with relatively low
surge activity such as intra-building applications defined in
Telcordia GR-1089-CORE
. This design is also suitable
for
ITU-T K.21
requirements.
Contact Microsemi Customer Applications if higher protection levels such as
Telcordia GR-1089-CORE
inter-building protection is required.
4.4.2 AC and DC Sense Components
The AC sense components are used to sense the AC signals on tip and ring. The AC sense is coupled the the TAC
and RAC pins by a series RC network comprised of a 0.022uF 100V X7R ceramic capacitor and a 10K
1% 0603
resistor. AC coefficients are designed around these values, therefore any deviation from the default values could
result in poor voice performance.
The DC sense componets are used to sense DC voltage on tip and ring and are requried for proper DC and ringing
performance. The DC sense couples tip and ring to the TDC and RDC pins though 1Meg 1% 1206 resistors. In
designs requiring high surge tolerance, the DC resistors may be two 499K
1% 1206 resistors in series. The DC
sense resistors MUST be 1M
1%. Because the DC sense connects in front of the surge protection, they are
exposed to high voltage conditions and therefore should be not smaller than 1206.
4.5 Buck-Boost Automatic Battery Switching Power Supply (BBABS)
The Shared Buck-Boost switching power supply for the
Le9642
device produces low and high battery voltages that
are shared by both channels. Refer to
for the schematic of the Buck-Boost ABS (BBABS) power supply for
ZLR964222L.The line module is designed to use 12 V (10 V to 14 V) nominal input and support both lines ringing a
5REN load at 50Vrms (with adaptive ringing enabled). Ambient operating temperature for this design is 0 to 70°C
(inside the box).
The BBABS supply uses a standard buck-boost power stage to create VBATL but adds a charge pump doubler
circuit to create VBATH. The nominal value of VBH is (2 x VBL) + VSW - 2. For a -25V VBL, the nominal value of
VBH will be -60V. The BBABS supply is dynamically adjusted depending on the state of the FXS interface. VBL is
set to -25V (VBH -60V) except when a line is in ringing state. In this case VBL is raised to -35 to -40V (depending
on required ringing level) which makes VBH is -80 to -90V. During the ringing period the non-ringing line will
dissipate extra power, but this is only a short term event.
The BBABS supply operates at a fixed switching frequency of 128kHz for medium and high power states. Low
power mode operates at 24kHz. The maximum allowed duty cycle in high power mode is 77%.
The peak current limit is set by RLIM
1-4
, 0.25
, in combination with RCS1 and RTH1. By adjusting RTH1, the peak
inductor current limit can be set. The Le9642 includes a cycle-by-cycle current limit to prevent inductor saturation.
The current limit threshold is usually set to a level slightly above the required peak switch current of the power
supply, or rated saturation current of the inductor.
Power supply input current is averaged by the input capacitor CSW
1
. The reference design uses a 220uF 25V low
impedance capacitor for CSW1. A smaller capacitor could be used if there is other bulk capacitance on the board
near the FXS circuit. It is recommended that additional bulk low impedance capacitance be used within the system
to minimize the effects of the switching supply. Many systems use the same 12 V input to supply other system
