BT8376EPF datasheet(108/323 Pages) CONEXANT

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2.0 Circuit Description

Bt8370/8375/8376

2.9 Transmit Line Interface Unit

Fully Integrated T1/E1 Framer and Line Interface

2-80

Conexant

N8370DSE

2.9.4 Line Driver

The line driver provides current drive to the low-power, bipolar analog signal

from the transmit Digital-to-Analog Converter (DAC). The transmit DAC

converts the coded pulse shape values to properly scaled, analog bipolar signals

which drive the line transformer. The coded pulse shape values can optionally be

filtered during T1 Longhaul by LBO filter options. The internal sensing circuits

limit the drive current to less than 50 mA when a shorted line condition is

detected for more than 48 transmitted pulses. Normal drive levels are restored

when the short is removed. Typically, this is caused by a transmit cable short

circuit or a transient transmission line current surge. The activation sets the

TSHORT bit in Alarm 2 Status register [ALM2; addr 048].

2.9.4.1 Termination

Impedance

If an external termination resistor is used, the TERM bit in TLIU_CR must be set.

The transformer setting must also be set via the TURNS bit in TLIU_CR. An

external termination resistor can be used only if the transformer turns ratio is

1:1.36. The external termination resistor (RTerm) is placed in parallel across

XTIP/XRING, as illustrated in Figure 2-45.

Figure 2-45. External Termination Resistor Placement

XTIP

XRING

RLoad

VOUT

RTerm

Cable

1:N

Bt8370

BT8376EPF Datasheet(PDF) 108 Page - Synaptics Incorporated.