UPS604 Datasheet, PDF (7/12 Pages) Unisonic Technologies – HIGH PERFORMANCE CURRENT MODE POWER SWITCH

UPS604
LINEAR INTEGRATED CIRCUIT
„ FUNCTIONAL DESCRIPTION(Cont.)
(2) Switch Frequency Set
The maximum switch frequency is set to 68kHz. Switch frequency is modulated by output power P_OUT during IC
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
load condition, The IC will operate at from Burst mode to Reducing Frequency Mode. The relation curve between f_SW
and P_OUT/P_{OUT (MAX)} as followed FIG.4.
FIG.4 The relation curve between f_SW and relative output power P_OUT/ P_{OUT (MAX)}
(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.
Fosc
(kHz)
F_{SW_MAX}
F_{SW_TYP}
F_{SW_MIN}
T_Hopping
FIG.5 Frequency Hopping
t(ms)
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
7 of 12
QW-R119-006.E