HC55151 datasheet(24/35 Pages) INTERSIL

24 / 35 page

4-24

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 VCC/2 reference. Thus, DC blocking

capacitors are once again required. By using the PTG pin of

the UniSLIC14 and the externally supplied VCC/2 reference

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

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 (RPS) 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 RPS. This occurs because RPS is in series with the

loop current and the negative supply.

A mathematical veriﬁcation follows:

Given: VBH = VBL = -48V, Loop current = 30mA, RL (load

across tip and ring) = 600

Ω, Quiescent battery power =

(48V) (0.8mA) = 38.4mW, Quiescent VCC power = (5V)

(2.7mA) = 13.5mW, Power sharing resistor = 600

Ω.

1. Without power sharing, the on-chip power dissipation

would be 952mW (Equation 41).

2. With power sharing, the on-chip power dissipation is

412mW (Equation 42). A power redistribution of 540mW.

On-chip power dissipation without power sharing resistor.

On-chip power dissipation with 600

Ω power sharing resistor.

The design trade-off in using the power sharing resistor is

loop length vs on-chip power dissipation.

FIGURE 24A.

FIGURE 24B.

FIGURE 24. INTERFACE TO DUAL AND SINGLE SUPPLY

CODECs

VTX

A = 1

VRX

UniSLIC14

VOUT

DUAL SUPPLY

CODEC/FILTER

GND

-5V

VTX

500K

PTG

A = 1

500K

VRX

UniSLIC14

SINGLE SUPPLY

CODEC/FILTER

GND

DSP

VIN

VOUT

VREF

() 30mA

() 38.4mW 13.5mW

() 30mA

()

–

(EQ. 41)

952mW

() 30mA

() 38.4mW 13.5mW

(EQ. 42)

() 30mA

()

–

() 30mA

()

–

412mW

FIGURE 25. POWER SHARING (SINGLE SUPPLY SYSTEMS)

VTX

VRX

UniSLIC14

TIP

RING

VBL

VBH

-48V

ON SHORT LOOPS, THE

MAJORITY OF CURRENT

FLOWS OUT THE VBL PIN

RPS

HC55120, HC55121, HC55130, HC55131, HC55140, HC55141, HC55142, HC55143, HC55150, HC55151

HC55151 Datasheet(PDF) 24 Page - Intersil Corporation

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