L3234 datasheet(14/26 Pages) STMICROELECTRONICS

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OPERATING MODES

Stand-By (SBY = 1 and RNG = 0)

In Stand-By mode the L3235 limits the DC Loop

current to 3 mA.

In this mode all the AC circuits are active and all

the AC characteristics are the same as in Active

Mode.

Also the two Line Current detectors (hook and

GND key) are active but due to the loop current

limited to 3 mA they will not be activated.

This mode is useful in emergency condition when

it is very important to limits the system power dis-

sipation.

Ringing Mode (SBY = 0 and RNG = 1)

When ringing mode is selected ”CS” pin is set to

1 in order to activate the L3234 ringing injector.

See L3234 for detailed description.

Ring trip is detected by means of the same inter-

nal circuitry used for off-hook detection.

An off-hook delay time lower than 1

⁄2 FRING should

be selected. (see ext. components list).

When ring trip is detected ”CS” is automatically

set to ”0” allowing in this way a quick ringing dis-

connection.

After Ring trip detection the Card Controller must

set the L3235 in active mode to remove the inter-

nal latching of the ”CS” information.

Active mode (SBY = 0 and CS1 = 0)

In Active mode the L3235 has the DC charac-

teristic show in Fig.13

The DC characteristics of L3235 has two different

feeding conditions:

1)Current Limiting Region : (short loop) the DC

impedance of the SLIC is very high (>20

Kohm) therefore the system works as a cur-

rent generator. By the ext. resistor RLIM con-

nected at pin 19 it is possible to program limit-

ing current values from 20 mA to 70 mA.

2) Voltage source region (long loop).

The DC impedance of the L3235 is almost

equal to zero therefore the system works like

a voltage generator with in series the two ex-

ternal protection resistors Rp.

When a limiting current value higher than 40 mA

is programmed the device will automatically re-

duce to 40 mA the loop current for very short

loop.

This is done in order to limit the maximum power

dissipation in very short loop to values lower than

2W for the external transistor and lower than

0.5W for the L3235 itself.

This improve the system reliability reducing the

L3235 power dissipation and therefore the inter-

nal junction temperature.

Table 1.

OPERATING

MODE

INPUT PIN

LINE STATUS

OUTPUT PIN

SBY

RNG

0: ON HOOK

1: OFF HOOK

0: NO GND KEY

1: GND KEY ON

GDK

ACTIVE

RINGING

0(*)

STAND-BY

(*)This status is latched and doesn’t change until RNG turn to 0

L3234 - L3235

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