HC55120CM INTERSIL [Intersil Corporation], HC55120CM Datasheet - Page 21

• Manufacturer Part Number: HC55120CM
• Description: Low Power Universal SLIC Family
• Manufacturer: INTERSIL [Intersil Corporation]

Supervisory Functions
Switch Hook Detect Threshold
The Switch Hook Detect Threshold is programmed with a
single external resistor (R
low when an off hook condition is detected.
Ground Key Detect Threshold
The Ground Key Detect Threshold is set internally and is not
user programmable.
Ringing the Phone
The UniSLIC14 family handles all the popular ringing
formats with high or low side ring trip detection. High side
detection is possible because of the high common mode
range on the ring signal detect input pins (DT, DR). To
minimize power drain from the ring generator, when the
phone is not being rung, the sense resistors are typically
2M . This reduces the current draw from the ring generator
to just a few microamps.
When the subscriber goes off hook during ringing, the
UniSLIC14 family automatically releases the ring relay and
DC feed is applied to the loop. The UniSLIC14 family has
very low power dissipation in the on hook active mode. This
enables the SLIC (during the ring cadence) to be powered
up in the active state, avoiding unnecessary powering up
and down of the SLIC. The control logic is designed to
facilitate easy implementation of the ring cadence, requiring
only one bit change to go from active to ringing and back
again.
DT, DR AND RRLY INPUTS
Ring trip detection will occur when the DR pin goes more
positive than DT by approximately 4V.
The ring relay driver pin, RRLY, has an internal clamp
between it’s output and ground. This eliminates the need to
place an external snubber diode across the ring relay.
Reducing Impulse Noise During Ringing
With an increase in digital data lines being installed next to
analog lines, the threat from impulse noise on analog lines is
increasing. Impulse noise can cause large blocks of high
speed data to be lost, defeating most error correcting
techniques. The UniSLIC14 family has the capability to
reduce impulse noise by closing the ring relay at zero
voltage and opening the ring relay at zero current.
CLOSING THE RING RELAY AT ZERO VOLTAGE
Closing the ring relay at zero voltage is accomplished by
providing a ring sync pulse to the RSYNC_REV pin. The ring
sync pulse is synchronized to go low at the zero voltage
crossing of the ring signal. The resistor R1 in Figure 18 limits
the current into the RSYNC_REV pin. If a particular polarity
reversal time is required, then make R1 equal to the
calculated value in Equation 39. If a specific polarity reversal
time is not desired, R1 equal to 50k is suggested.
HC55120, HC55121, HC55130, HC55131, HC55140, HC55141, HC55142, HC55143, HC55150, HC55151
D
4-21
). The output of the SHD pin goes
The RSYNC_REV pin is designed to allow the ring sync
pulse to be present at all times. There is no need to gate the
ring sync pulse on and off. The logic control for the
RSYNC_REV pin cannot be an open collector. It must be
high (push-pull logic output stage / pull up resistor to VCC),
low or being clocked by the ring sync pulse. When the
RSYNC_REV pin is high the ring relay pin is disabled. When
the RSYNC_REV pin is low the ring relay pin is activated the
instant the logic code for ringing is applied.
OPENING THE RING RELAY AT ZERO CURRENT
The ring relay is automatically opened at zero current by the
SLIC. The SLIC logic requires zero ringing current in the
loop and either a valid switch hook detect (SHD) or a change
in the operating mode (cadence of the ringing signal) to
release the ring relay.
If the subscriber goes off hook during ringing, the SHD
output will go low. An internal latch will sense SHD is low and
disable the ring relay at zero ringing current. This prevents
the ring signal from being reapplied to the line. To ring the
line again, the SLIC must toggle between logic states. (Note:
The previous state can not be the Reverse Active State. In
the reverse state, the voltage on the CRT_REV_LVM
capacitor will activate an internal latch prohibiting the ringing
of the line.
Figure 19 shows the sequence of events from ringing the
phone to ring trip. The ring relay turns on when both the
ringing code and ring sync pulse are present (A). SHD is
high at this point. When the subscriber goes off hook the
SHD pin goes low and stays low until the ringing control
code is removed (B). This prevents the SHD output from
pulsing after ring trip occurs. At the next zero current
crossing of the ring signal, ring trip occurs and the ring relay
releases the line to allow loop current to flow in the loop (C).
FIGURE 18. REDUCING IMPULSE NOISE USING THE
UniSLIC14
RSYNC_REV
RSYNC_REV PIN AND SETTING THE POLARITY
REVERSAL TIME
24
50k
R
1
INPUT FOR THE
RING SYNC PULSE
0V
5V