HC55120_06 Datasheet, PDF (25/36 Pages) Intersil Corporation – Low Power Universal SLIC Family

HC55120, HC55121, HC55130, HC55140, HC55142, HC55143, HC55150
Overload Levels and Silent Polarity Reversal
The pulse metering signal and voice are simultaneously
transmitted, and therefore require additional overhead to
prevent distortion of the signal. Reference section “Off hook
Overhead Voltage” to account for the additional pulse
metering signal requirements.
I_X
5
OUTPUT BUFFER
RA
500K
RB
500K
-
+
A=1
V_TX
PTG
-
V_TX
+
I_X
125K
30.1K
R _{added to}
bottom of
V_SPM board
UniSLIC14
150pF
C _{added to}
+
V_ttx
bottom of
-
board
FIGURE 23. PULSE METERING WITH TRANSHYBRID
BALANCE
Most of the SLICs in the UniSLIC14 family feature full
polarity reversal. Full polarity reversal means that the SLIC
can transmit, determine the status of the line (on hook and
off hook) and provide “silent” polarity reversal. Reference
Equation 39 to program the polarity reversal time.
Interface to Dual and Single Supply
CODECs
Great care has been taken to minimize the number of external
components required with the UniSLIC14 family while still
providing the maximum flexibility. Figures 24A, 24B) shows
the connection of the UniSLIC14 to both a dual supply
CODEC/Filter and a single supply DSP CODEC/Filter.
To eliminate the DC blocking capacitors between the SLIC
and the CODEC/Filter when using a dual supply
CODEC/Filter, both the receive and transmit leads of the
SLIC are referenced to ground. This leads to a very simple
SLIC to CODEC/Filter interface, as shown in Figure 24A.
When using a single supply DSP CODEC/Filter the output
and input of the CODEC/Filter are no longer referenced to
ground. To achieve maximum voltage swing with a single
supply, both the output and input of the CODEC/Filter are
referenced to its own V_CC/2 reference. Thus, DC blocking
capacitors are once again required. By using the PTG pin of
the UniSLIC14 and the externally supplied V_CC/2 reference
of the CODEC/Filter, one of the DC blocking capacitors can
be eliminated (Figure 24B).
-
V_TX
+
A=1
V_RX
UniSLIC14
-
+
DUAL SUPPLY
CODEC/FILTER
V_OUT
5V
GND
-5V
FIGURE 24A.
500K
500K
-
+
A=1
V_TX
PTG
V_RX
UniSLIC14
V_IN
V_REF SINGLE SUPPLY
DSP
CODEC/FILTER
V_OUT
5V
GND
FIGURE 24B.
FIGURE 24. INTERFACE TO DUAL AND SINGLE SUPPLY
CODECs
Power Management
The UniSLIC14 family provides two distinct power
management capabilities:
Power Sharing and Battery Selection
Power Sharing
Power sharing is a method of redistributing the power away
from the SLIC in short loop applications. The total system
power is the same, but the die temperature of the SLIC is
much lower. Power sharing becomes important if the
application has a single battery supply (-48V on hook
requirements for faxes and modems) and the possibility of
high loop currents (reference Figure 25). This technique
would prevent the SLIC from getting too hot and thermally
shutting down on short loops.
The power dissipation in the SLIC is the sum of the smaller
quiescent supply power and the much larger power that
results from the loop current. The power that results from the
loop current is the loop current times the voltage across the
SLIC. The power sharing resistor (R_PS) reduces the voltage
across the SLIC, and thereby the on-chip power dissipation.
The voltage across the SLIC is reduced by the voltage drop
across R_PS. This occurs because R_PS is in series with the
loop current and the negative supply.
A mathematical verification follows:
25
FN4659.13
June 1, 2006