CPC7593

R05

www.ixysic.com

19

2.3.8 Alternate Break-Before-Make Operation Logic Table (Ringing to Talk Transition)

2.4 Data Latch

The CPC7593 has an integrated transparent data

latch. The latch enable operation is controlled by TTL

logic input levels at the LATCH pin. Data input to the

latch are via the input pins, while the output of the data

latch are internal nodes used for state control. When

the LATCH enable control pin is at logic 0 the data

latch is transparent and the input data control signals

flow directly through the latch to the state control

circuitry. A change in input will be reflected by a

change in switch state. Whenever the LATCH enable

control pin is at logic 1, the latch is active and data is

locked. Subsequent input changes will not result in a

change to the control logic or affect the existing switch

state.

Switches will remain in the state they were in when the

LATCH pin changes from logic 0 to logic 1 and will not

respond to changes in input as long as the latch is at

output control functions are affected by the latch

function. Internal thermal shutdown control and

state of the LATCH enable input.

pin indicates the status of the thermal shutdown

circuitry. Typically, during normal operation, this pin will

create excess thermal loading the CPC7593 will enter

thermal shutdown and a logic low will be output.

CPC7593 into the “All-Off” state by simply pulling the

input low via an open-collector type buffer. Using a

standard output with an active logic high drive

capability will sink the pull-up current resulting in

unnecessary power consumption.

Use of a standard output buffer with an active high

drive capability will not disable the thermal shutdown

mechanism. The ability to enter thermal shutdown

during a fault condition is independent of the

The CPC7593’s internal pull up has a nominal value of

16

A.

2.6 Ringing Switch Zero-Cross Current Turn Off

After the application of a logic input to turn SW4 off,

the ringing switch is designed to delay the change in

state until the next zero-crossing. Once on, the switch

requires a zero-current cross to turn off, and therefore

should not be used to switch a pure DC signal. The

switch will remain in the on state no matter the logic

input until the next zero crossing. These switching

characteristics will reduce and possibly eliminate

overall system impulse noise normally associated with

ringing switches. See application note AN-144, Impulse

Noise Benefits of Line Card Access Switches for more

information. The attributes of ringing switch SW4 may

make it possible to eliminate the need for a zero-cross

switching scheme. A minimum impedance of 300

 in

series with the ringing generator is recommended.

State

Latch

Timing

Break

Switches

Ringing

Return

Switch

(SW3)

Ringing

Switch

(SW4)

Test

Switches

Ringing

1

0

0

0

Z

-

Off

On

On

Off

All-off

1

0

1

X0

Hold this state for at least

one-half of ringing cycle.

SW4 waiting for zero current

to turn off.

Off

Off

On

Off

Break-

Before-

Make

00

0

Zero current has occurred.

SW4 has opened

Off

Off

Off

Off

Talk

0

0

0

0

Z

Break switches close.

On

Off

Off

Off