HC55120CM INTERSIL [Intersil Corporation], HC55120CM Datasheet - Page 24

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

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
capacitors are once again required. By using the PTG pin of
the UniSLIC14 and the externally supplied V
of the CODEC/Filter, one of the DC blocking capacitors can
be eliminated (Figure 24B).
Power Management
The UniSLIC14 family provides two distinct power
management capabilities:
Power Sharing and Battery Selection
HC55120, HC55121, HC55130, HC55131, HC55140, HC55141, HC55142, HC55143, HC55150, HC55151
FIGURE 24. INTERFACE TO DUAL AND SINGLE SUPPLY
500K
500K
UniSLIC14
UniSLIC14
+
+
-
-
A = 1
A = 1
CODECs
V
V
V
V
PTG
RX
RX
TX
TX
CC
FIGURE 24A.
FIGURE 24B.
/2 reference. Thus, DC blocking
4-24
V
V
V
OUT
REF
OUT
V
IN
+
-
SINGLE SUPPLY
CODEC/FILTER
CODEC/FILTER
DUAL SUPPLY
DSP
CC
/2 reference
5V
GND
-5V
5V
GND
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
across the SLIC, and thereby the on-chip power dissipation.
The voltage across the SLIC is reduced by the voltage drop
across R
loop current and the negative supply.
A mathematical verification follows:
Given: V
across tip and ring) = 600 , Quiescent battery power =
(48V) (0.8mA) = 38.4mW, Quiescent VCC power = (5V)
(2.7mA) = 13.5mW, Power sharing resistor = 600 .
On-chip power dissipation without power sharing resistor.
On-chip power dissipation with 600 power sharing resistor.
The design trade-off in using the power sharing resistor is
loop length vs on-chip power dissipation.
PD
PD
PD
PD
1. Without power sharing, the on-chip power dissipation
2. With power sharing, the on-chip power dissipation is
FIGURE 25. POWER SHARING (SINGLE SUPPLY SYSTEMS)
would be 952mW (Equation 41).
412mW (Equation 42). A power redistribution of 540mW.
=
=
=
=
ON SHORT LOOPS, THE
MAJORITY OF CURRENT
FLOWS OUT THE V
–
952mW
412mW
V
V
BH
BH
R
BH
PS
L
. This occurs because R
= V
30mA
30mA
30mA
BL
= -48V, Loop current = 30mA, R
2
+
+
–
BL
38.4mW
38.4mW
RING
R
PIN
PS
TIP
30mA
+
+
UniSLIC14
-48V
13.5mW
13.5mW
V
BL
R
2
PS
PS
PS
) reduces the voltage
is in series with the
-48V
V
–
BH
RL 30mA
L
V
V
(EQ. 41)
(EQ. 42)
RX
(load
TX
2