Using the ISL6401 RSLIC PWM Controller
Evaluation Board
®
Application Note
October 2003
AN1082
Author: Manisha Pandya, Jason Houston
In a telephone loop, the subscriber is connected to the
network via 2 wires, commonly known as Tip and Ring. The
actual digital telecommunications trunk line however,
operates on 4-wires; two of which are allocated for
transmitting and two for receiving. This 2 to 4-wire interface
consists of the SLIC and CODEC. A SLIC is the primary
interface between the 4-wire (ground referenced) low
voltage switch environment and the 2 wire (floating) high
voltage loop environment. It performs a number of important
functions including Battery feed, Overvoltage protection,
Ringing, Signaling, Coding, Hybrid Balancing and also
Testing.
Functional Description
The ISL6401 pulse width modulating (PWM) current mode
controller is designed for a wide range of DC-DC conversion
applications including boost, flyback, and isolated output
configurations. The device is optimized to provide high
performance, low-cost solution for Ringing SLIC (RSLIC)
Ring (Vbh) and Talk (Vbl) power supplies in VoIP
applications. The IC features an integrated inverter that is
ideal for generating negative output voltage like RSLIC Ring
Vbh (-72V) and Talk Vbl (-24V), -48V for IP Phones, -5V and
-15V for DSL CO line drivers. The output voltages are
adjusted with an external voltage divider.
The Ringing SLIC (RSLIC) typically requires two high
voltage power supply inputs. The first is a tightly regulated
voltage around -24V or -48V for off-hook signal transmission.
The second is a loosely regulated -70 to -100V for ring tone
generation. When the switch hook is released the phone
puts approximately 200Ω of resistance across the phone
terminals. Intersil RSLICs feature internal current limiting so
this load is not presented to the power supply. However, not
all of the SLICs available in the market offer this feature and
the power supply is expected to maintain output during the
remainder of the ring cycle. Once voice transmission begins,
the SLIC, in many cases, requires a lower voltage input to
establish a 20-25mA current loop. The loop feeds the 200Ω,
protection resistors, and line resistances within the phone. In
some cases, the lower supply and higher supply voltage are
combined and the SLIC runs from a compromise voltage of
approximately -53V.
Peak current mode control architecture effectively handles
Ring trip transients and provides inherent over-current
protection. Flyback topology allows the operation close to
50% duty cycle, offering optimum transformer utilization, low
ripple current and less stress on input/output capacitors.
Internal soft start minimizes start-up stress without any
external components. The switching frequency can be
programmed from 50kHz to 600kHz or alternatively the
internal oscillator can be locked to an external clock fed at
SYNC input for noise sensitive applications. A logic level
shutdown input is included, which reduces supply current to
55µA in the shutdown mode. DC-DC conversion efficiency is
optimized by use of a low current sense voltage.
For a detailed functional description, complete specifications
and component selection guidelines, please refer to the
ISL6401 Data Sheet, Intersil Corporation, File No. FN9007,
available on Intersil’s website, http://www.intersil.com/
The specifications below are for a 4-line requirement with
5 REN per line
Application Information
TABLE 1. TYPICAL POWER SUPPLY REQUIREMENT FOR
VoIP RESIDENTIAL GATEWAY
As worldwide demand for inexpensive Voice over Internet
Protocol telephony grows, so will the need for Integrated
Circuits that are specialized to enable compatibility between
new telephony systems and older telephones based on
analog standards. Analog ring signal generation and off
hook loop current supply are two analog functions that are
performed by Subscriber Line Interface Circuits (SLICs).
This application note discusses the special power supply
implementation to generate the high negative voltages
needed by SLICs.
PARAMETER
Input Voltage
REQUIREMENT
5 or 12 volts
Output Power
Efficiency
3 to 10 watts
80 to 90%
Output Voltages
-24V, -72 to -100V and/or
-48V
-24V Requirements (4 lines)
-72V Requirements (4 lines)
Regulation: ±5% Maximum
Output Current: 0.10A Ripple:
Less than 0.25Vpp
Overview of Telephone loop system
Traditionally a telephone network consists of a circuit
between the subscriber and the central office. However, the
advent of new high speed digital technologies have created
the need to control and manage the functions of the phone
locally as opposed to the central office. In both instances the
principals governing the operation of the phone loop are
essentially the same.
Regulation: ±10% Maximum
Output Current: 0.10A Ripple:
Less than 1Vpp
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1
1-888-INTERSIL or 321-724-7143 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2003. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
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