IP phones requ
ire that data and power on the same cable. A high voltage power source of 48V is required, to reduce the voltage drop in the cable. This application note describes a DC-DC
step-up converter that can deliver the required 48V supply, from the 12V source typically available in the host of such systems. It delivers low-voltage ripple, essential to prevent data errors.
Isolation is provided by the 12V power-supply. For a system with one or two ports, a custom-designed DC-DC converter is more suitable than power modules, or "bricks", since bricks tend to be bulkier and a
re more expensive. They are more suitable for systems that support more ports, where the power demand is higher.
The MAX668 is an excellent choice for the design of a DC-DC step-up converter. The MAX668, a current-mode controller, operates in the PWM mode at medium and heavy loads, providing high-efficiency and low-noise. With power levels greater than 20W, efficiencies of more than 90% are achievable.
The circuit of Figure 2 has a 48V output and can deliver current up to 1 ampere, enough to power a system with two ports. L2 and C8 form a post filter to further reduce the ripple voltage below 50mV. A switching frequency of 400kHz was chosen as a compromise between switching loss and the size of inductor L1. Increasing the value of gate resistance R7 will reduce EMI noise, but increase switching loss. A value of 2.0Ω was chosen. At an input voltage of 12V, a full load of 1 ampere and an output of 48V, the peak-peak voltage ripple was 36mv with a conversion efficiency of 91%.