L8575 datasheet(20/36 Pages) AGERE

20 / 36 page

L8575

Advance Data Sheet

Dual-Resistive, Low-Cost SLIC

March 1997

Lucent Technologies Inc.

Applications (continued)

Protection (continued)

Similar consideration to the individual resistor and total

module power capability should be given to full voltage

power faults, but taking into account the fault detector

will isolate the SLIC and resistor module after some

ﬁnite period of time. The fault detector indicates a fault

in the serial data output stream in 10 ns to 30 ms. Rec-

ognition and relay activation time need to be consid-

ered.

Tip/Ring Drivers

Each channel of the L8575 utilizes a current source for

the Tip/Ring driver. The driver is capable of sinking (but

not sourcing) up to 15 mA from the Tip (PT) while

swinging to within 4 V of ofﬁce battery (VBAT), and

sourcing (but not sinking) up to 15 mA to the Ring (PR)

while swinging to within 4 V of ground (AGND). Since

the current driver is not bidirectional, during transmis-

sion (powerup) each lead is biased at 5.6 mAdc.

Receive Interface

The receive interface circuitry couples the differential

signal on receive inputs IRP and VRN to the Tip/Ring

drivers. Input IRP is a low-impedance (<5

Ω) current

input while VRN is a high-impedance voltage input.

Internal feedback forces the voltage at IRP to be equal

to VRN such that a voltage applied to VRN causes a

current ﬂow out of IRP, which equals that voltage

divided by the impedance connected from IRP to

AGND (assuming the input voltage is referenced to

AGND).

The receive interface and Tip/Ring drivers provide a

current gain of 200, i.e., a differential output current

ﬂows from PT to PR which is 200 times the current

ﬂowing into IRP. The receive interface also provides a

level shift since the inputs, IRP and VRN, are refer-

enced to analog ground, while the outputs, PT and PR,

swing between AGND and VBAT. The receive interface

ensures that the input current is not converted to a

common-mode current at PT and PR.

Transmit Interface

The transmit interface circuitry interfaces the differen-

tial voltage on Tip and Ring to transmit output XMT. The

Tip/Ring differential voltage (both ac and dc) appears

on output XMT with a gain of 0.5.

The transmit interface uses an operational ampliﬁer

with four external resistors to perform a differential to

single-ended conversion. Output XMT is referenced to

ground (AGND). The longitudinal balance and gain

accuracy at XMT depends on the matching of the

external resistors.

Because a large dc potential exists at XMT, a capacitor

must be used to couple the ac signal to the low-voltage

codec circuitry. The operational ampliﬁer inputs are TS

and RS. These inputs are also used by the fault-detec-

tion circuitry to detect fault voltages on Tip or Ring. A

fault is detected when the magnitude of the voltage

across either dc feed resistor exceeds a nominal 36 V

(equivalent to approximately 4 W dissipation in either

resistor). A delay is provided (using an external capaci-

tor on pin CFLT) in the fault detector. This prevents

transients on Tip and Ring from tripping the fault detec-

tor when a fault is not actually present.

Battery Noise Cancellation

The battery noise cancellation circuit senses the ac

noise on the battery via the capacitor connected

from input CBN to VBAT. It then couples this noise,

180 degrees out of phase, to the Ring current driver

ampliﬁer. This cancels the battery noise that is coupled

to the Ring through the feed resistor connected to VBAT.

Additionally, it ensures longitudinal balance which

depends only on the matching of the battery feed resis-

tors by creating an ac ground at VBAT with respect to

signals on the Ring lead.

For the cancellation to operate properly, both the phase

and gain must be accurate. The battery noise cancella-

tion gain is a transconductance that is equal to 50

divided by resistor R9 on the thick ﬁlm resistor module

connected from RGBN to ground (AGND). This value

must be equal to the reciprocal of the dc feed resistor

(1/300

Ω), that is,

It is advantageous if the two resistors are matched and

tracked thermally, i.e., located on the same ﬁlm inte-

grated circuit (FIC).

-------

300

Ω

----------------

15 k

Ω

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