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HT0638

HT0638

Electronic Line Switch

General Description

The Supertex HT0638 is an integrated electronic line switch

controllable from four logic-level inputs. These devices are

intended as a replacement for mechanical hook switches and

relays in telephones, answering machines, and similar telecom-

munications equipment. They switch the positive (Tip) side of the

telephone line using control inputs that are referenced to the

negative (Ring) side of the line. In the off state, it can hold-off 375

volts. In the on state, the series resistance is 15Ω maximum.

These devices provide adjustable current limiting via an external

sense resistor. Three of the control inputs are general purpose

and one is an override input that forces the switch to an off state

regardless of the state of the other inputs. This override input is

useful for ‘flash’ and pulse dialing purposes. Internal pull-up/

down resistors allow the inputs to be driven from single-ended

sources, push-pull sources, or left unconnected.

The HT0638 is powered from the phone line and are especially

useful in applications where external power is not available. Due

to their low quiescent current, both of these devices exceed the

on-hook resistance requirements of EIA-470-A.

Features

❑ 15Ω maximum switch resistance

❑ Current limiting protection

❑ 4 logic control inputs

❑ Operates at 2V input

❑ 2µA maximum quiescent current at 100V

Applications

❑ Telephone handsets

❑ Fax machines

❑ Answering machines

❑ Auto-dial alarm systems

08/23/02

Supertex Inc. does not recommend the use of its products in life support applications and will not knowingly sell its products for use in such applications unless it receives an adequate "products liability

indemnification insurance agreement." Supertex does not assume responsibility for use of devices described and limits its liability to the replacement of devices determined to be defective due to

workmanship. No responsibility is assumed for possible omissions or inaccuracies. Circuitry and specifications are subject to change without notice. For the latest product specifications, refer to the

Supertex website: http://www.supertex.com. For complete liability information on all Supertex products, refer to the most current databook or to the Legal/Disclaimer page on the Supertex website.

For additional information, refer to application note

AN-H32.

Typical Application Circuit

Low

Voltage

Telecom

IC

TPG

HT0640

TP

CS

GND

HKS

LS1

LS2

DP

H=ON

L= OFF

Charge Pump

&

Logic Control

+

15V

-

Tip

Ring