8
Am79231 Data Sheet
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 Ca-
pacitor
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 Am79231 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 Am79231 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 Am79231 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
Ring Relay Driver
O
Collector connection for ring relay 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 Legerity 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 Am79231.
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.
VCC
+5 V Power Supply
Positive supply for low voltage analog and digital circuits in the Am79231.
VLB
Longitudinal Voltage
I
Sets the DC longitudinal voltage of the Am79231. It is the reference for the longitudinal control
loop. When the VLB pin is greater than VREF, the Am79231 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 Refer-
ence
I
The ISLAC chip provides this voltage which is used by the Am79231 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.
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