22

Am79R251

Table 2.

Mode Descriptions

Operating

Mode

Description

Disconnect

This mode disconnects both A and B output amplifiers from the AD and BD outputs. The

A and B amplifiers are shut down and the Am79R251 selects the low battery voltage at

the VBL pin. In the Disconnect state, the currents on IMT and ILG represent the voltages

on the SA and SB pins, respectively. These currents are scaled to produce voltages across

RMTi and RLGi of

and

, respectively.

Standby

The power amplifiers are turned off. The AD output is driven by an internal 250

Ω (typical)

resistor, which connects to ground. The BD output is driven by an internal 250

Ω (typical)

resistor, which connects to the high battery (BATH) at the VBH pin, through a clamp circuit,

which clamps at –50 V with respect to BGND. For VBH values above–55 V, the open-

circuit voltage, which appears at this output is ~VBH + 5 V. If VBH is below –55 V, the

voltage at this output is –50 V. The battery selection for the balance of the circuitry on the

chip is VBL. Line supervision remains active. Current limiting is provided on each line to

limit power dissipation under short-loop conditions as specified in the “Am79R251 Current-

Limit Behavior” section. In external ringing, the standby ISLIC state is selected.

Tip Open

In this mode, the AD (Tip) lead is opened and the BD (Ring) lead is connected to a clamp,

which operates from the high battery on VBH pin and clamps to approximately –50 V with

respect to BGND through a resistor of approximately 250

Ω (typical). The battery selection

for the balance of the circuitry on the chip is VBL. To prevent excessive power dissipation,

the current in the Ring lead is limited by an internal current source to 30 mA.

Active High

Battery

In the Active High Battery mode, battery connections are connected as shown in

Table 1. Both output amplifiers deliver the full power level determined by the programmed

DC-feed conditions. Active High Battery mode is enabled during a call in applications

when a long loop can be encountered.

Active Low

Battery

Both output amplifiers deliver the full power level determined by the programmed DC-feed

conditions. VBL, the low negative battery, is selected in the Active Low Battery mode. This

is typically used during the voice part of a call.

Active Boosted

Battery

In the Active Boosted Battery mode, battery connections are as shown in Table 1. Both

output amplifiers deliver the full power level determined by the programmed DC-feed

conditions. Active Boosted Battery mode is enabled during a call in applications when an

extended loop can be encountered.

Active Internal

Ringing

In the Internal Ringing mode, the Am79R251 selects the battery connections as shown

in Table 1. When using internal ringing, both the AD and BD output amplifiers deliver the

ringing signal determined by the programmed ringing level.

On-Hook

Transmission

(OHT), Fixed

Longitudinal

Voltage

In the On-Hook Transmission, Fixed Longitudinal Voltage mode, battery connections are

as shown in Table 1. The longitudinal voltage is fixed at –30 V to allow compliance with

safety specifications for some classes of products.

V

SA

400

----------

V

SB

400

----------