11

FN4539.3

shutdown in the presence of a persistent thermal alarm.

Switching between high and low battery will have no effect

during power denial.

Functionality

During power denial, both the Tip and Ring amplifiers are

disabled, representing high impedances. The voltages at

both outputs are near ground.

Thermal Shutdown

In the event the safe die temperature is exceeded, the ALM

output will go low and DET will go high and the part will

automatically shut down. When the device cools, ALM will

go high and DET will reflect the loop status. If the thermal

fault persists, ALM will go low again and the part will

shutdown. Programming power denial will permanently

shutdown the device and stop the self cooling cycling.

Battery Switching

Overview

The integrated battery switch selects between the high

battery (VBH) and low battery (VBL). The battery switch is

controlled with the logic input BSEL. When BSEL is a logic

high, the high battery is selected and when a logic low, the

low battery is selected. All operating modes of the device will

operate from high or low battery except forward loop back.

Functionality

The logic control is independent of the operating mode

decode. Independent logic control provides the most

flexibility and will support all application configurations.

When changing device operating states, battery switching

should occur simultaneously with or prior to changing the

operating mode. In most cases, this will minimize overall

power dissipation and prevent glitches on the DET output.

The only external component required to support the battery

switch is a diode in series with the VBH supply lead. In the

event that high battery is removed, the diode allows the

device to transition to low battery operation.

Low Battery Operation

All off hook operating conditions and ringing should use the

low battery. The prime benefit will be reduced power

dissipation. The typical low battery for the device is -24V.

However this may be increased to support longer loop

lengths or high loop current requirements. Standby

conditions may also operate from the low battery if MTU

compliance is not required, further reducing standby power

dissipation.

High Battery Operation

The high battery should be used for standby conditions

which must provide MTU compliance. During standby

operation the power consumption is typically 40 mW with -

48V battery. If standby requirements do not require high

battery operation, then a lower battery will result in lower

standby power.

Uncommitted Switch

Overview

The uncommitted switch is a three terminal device designed

for flexibility. The independent logic control input, SWC,

allows switch operation regardless of device operating

mode. The switch is activated by a logic low. The positive

and negative terminals of the device are labeled SW+ and

SW- respectively.

Relay Driver

The uncommitted switch may be used as a relay driver by

connecting SW+ to the relay coil and SW- to ground. The

switch is designed to have a maximum on voltage of 0.6V

with a load current of 45mA.

Since the device provides the ringing waveform, the relay

functions which may be supported include subscriber

disconnect, test access or line interface bypass. An external

snubber diode is not required when using the uncommitted

switch as a relay driver.

Test Load

The switch may be used to connect test loads across Tip

and Ring. The test loads can provide external test

termination for the device. Proper connection of the

uncommitted switch to Tip and Ring is shown below.

The diode in series with the test load blocks current from

flowing through the uncommitted switch when the polarity of

the Tip and Ring terminals are reversed. In addition to the

reverse active state, the polarity of Tip and Ring are

reversed for half of the ringing cycle. With independent logic

control and the blocking diode, the uncommitted switch may

be continuously connected to the Tip and Ring terminals.

FIGURE 9. EXTERNAL RELAY SWITCHING

RELAY

SW+

SW-

SWC

+5V

FIGURE 10. TEST LOAD SWITCHING

RING

TIP

TEST

LOAD

SW+

SW-

SWC

HC5549