UPS703

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

7 of 12

www.unisonic.com.tw

QW-R119-015.C

FUNCTIONAL DESCRIPTION(Cont.)

(2) Switch Frequency Set

operating. At no load or light load condition, most of the power dissipation in a switching mode power supply is from

switching loss on the MOSFET transistor, the core loss of the transformer and the loss on the subber circuit. The

magnitude of power loss is in proportion to the number of switching events within a fixed period of time. So lower

switch frequency at lower load, which more and more improve IC’s efficiency at light load. At from no load to light

(3) Internal Synchronized Slope Compensation

Built-in slope compensation circuit adds voltage ramp onto the current sense input voltage for PWM

generation, this greatly improves the close loop stability at CCM and prevents the sub-harmonic oscillation and thus

reduces the output ripple voltage.

(4) Frequency Hopping For EMI Improvement

The Frequency hopping is implemented in the IC; there are two oscillators built-in the IC. The first oscillator

is to set the normal switching frequency; the switching frequency is modulated with a period signal generated by the

2nd oscillator. The relation between the first oscillator and the 2nd oscillator as followed FIG.5. So the tone energy is

evenly spread out, the spread spectrum minimizes the conduction band EMI and therefore eases the system design

in meeting stringent EMI requirement.

t(ms)

Fosc

(kHz)

FIG.5 Frequency Hopping