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.
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.

Application Information
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.