8

Am79R251

PIN DESCRIPTIONS

Pin

Pin Name

I/O

Description

AD, BD

A, B Line Drivers

O

Provide the currents to the A and B leads of the subscriber loop.

BGND

Ground

Ground return for high and low battery supplies.

CREF

+3.3 VDC

VCCD reference. It is the digital high logic supply rail, used by the ISLIC to ISLAC interface.

GND

Ground

Analog and digital ground return for VCC.

HPA, HPB

High-Pass Filter

Capacitor

O

These pins connect to CHP, the external high-pass filter capacitor that separates the DC

loop-voltage from the voice transmission path.

ILG

Longitudinal Current

Sense

O

ILG is proportional to the common-mode line current (IAD – IBD), except in disconnect mode,

where ILG is proportional to the current into grounded SB.

IMT

Metallic Current Sense

O

IMT is proportional to the differential line current (IAD + IBD), except in disconnect mode,

where IMT is proportional to the current into grounded SA.The Am79R251 indicates thermal

overload by pulling IMT to CREF.

LD

Register Load

I

The LD pin controls the input latch and responds to a 3-level input. When the LD pin is a

logic 1 ( >(Vref + 0.3V) ), the logic levels on P1–P3 latch into the Am79R251 control register

bits that operate the mode-decoder. When the LD pin is a logic 0 ( <(Vref – 0.3V) ), the logic

levels on P1–P3 latch into the Am79R251 control register bits that control the relay drivers

(RD1–RD3). When the LD pin level is at ~VREF, the control register contents are locked.

P1–P3

Control Bus

I

Inputs to the latch for the operating-mode decoder and the relay-drivers.

R1

Relay 1 Driver

O

Collector connection for relay 1 driver. Emitter internally connected to BGND.

R2

Relay 2 Driver

O

Collector connection for relay 2 driver. Emitter internally connected to RYE

R3

Relay 3 Driver

O

Collector connection for relay 3 driver. Emitter internally connected to RYE.

RSN

Receive Summing

Node

I

The metallic current between AD and BD is equal to 500 times the current into this pin.

Networks that program receive gain and two-wire impedance connect to this node. This input

is at a virtual potential of VREF.

RSVD

Reserved

This is used during AMD testing. In the application, this pin must be left floating.

RYE

Relay 2, 3 Common

Emitter

O

Emitter connection for R2 and R3. Normally connected to relay ground.

SA, SB

A, B Lead Voltage

Sense

I

Sense the voltages on the line side of the fuse resistors at the A and B leads. External sense

resistors, RSA and RSB, protect these pins from lightning or power-cross.

TMP,

TMN, TMS

Thermal Management

External resistors connected from TMP to TMS and TMN to VBL to offload excess power

from the Am79R251.

VBH

Battery (Power)

Connection to high-battery supply used for ringing and long loops. Connects to the substrate.

When only a single battery is available, it connects to both VBH and VBL.

VBL

Battery (Power)

Connection to low-battery supply used for short loops. When only a single battery is available,

this pin can be connected to VBH.

VBP

Positive Battery

(Power)

Used in Ringing State and for Extended Loop operation.

VCC

+5 V Power Supply

Positive supply for low voltage analog and digital circuits in the Am79R251.

VLB

Longitudinal Voltage

I

Sets the DC longitudinal voltage of the Am79R251. It is the reference for the longitudinal

control loop. When the VLB pin is greater than VREF, the Am79R251 sets the longitudinal

voltage to a voltage approximately half-way between the positive and negative power supply

battery rails. When the VLB pin is driven to levels between 0V and VREF, the longitudinal

voltage decreases linearly with the voltage on the VLB pin.

VREF

1.4 V Analog

Reference

I

The ISLAC chip provides this voltage which is used by the Am79R251 for internal reference

purposes. All analog input and output signals interfacing to the ISLAC chip are referenced

to this pin.

VSAB

Loop Voltage

O

Scaled-down version of the voltage between the sense points SA and SB on this pin.

VTX

4-Wire Transmit Signal

O

The voltage between this pin and VREF is a scaled down version of the AC component of

the voltage sensed between the SA and SB pins. One end of the two-wire input impedance

programming network connects to VTX. The voltage at VTX swings positive and negative

with respect to VREF.