US1651

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

5 of 7

www.unisonic.com.tw

QW-R129-005.a



OPERATION DESCRIPTION (Cont.)

Constant Current Operation

When INV lower than 1.25V, the system works in CC control, In CC operation, the CC point and maximum output

power can be externally adjusted by external current sense resistor Rcs. The larger Rcs, the saller CC point is, and

the smaller output power becomes, and vice versa as shown in Fig.2.

In charger applications, a discharged battery charging starts in the CC portion of the curve until it is nearly full

charged and smoothly switches to operate in CV portion of the curve.

In an AC/DC adapter, the normal operation occurs only on the CV portion of the curve. The CC portion provides

added cycle-by-cycle current limiting protection.

Extended Burst Mode Operation

At light load or zero load condition, most of the power dissipation in a switching mode power supply is from

switching loss, the core loss of the transformer and the loss on the snubber circuit. The magnitude of power loss is in

proportion to the switching frequency. Lower switching frequency leads to the reduction on the power loss and thus

conserves the energy.

The UTC US1651 self adjusts the switching frequency according to the loading condition. The switch frequency is

reduced at light/no load condition to improve the conversion efficiency. At light load/no load condition, the output of

the Error amplifier (EA) drops below the burst mode threshold level and device enters Burst Mode control. The

frequency control also eliminates the audio noise at any loading conditions.

Operation switching frequency

The switching frequency of UTC US1651 is internally set and adjusted according to CC/CV operation. No

external frequency setting components are required.

Frequency shuffling for EMI improvement

The frequency Shuffling/jittering (switching frequency modulation) is implemented in UTC US1651. The

oscillation frequency is modulated with a pseudo random source so that the tone energy is spread out. The spread

spectrum minimizes the conduction band EMI and therefore eases the system design.

Current Sensing and Leading Edge Blanking

Cycle-by-cycle current limiting is offered in UTC US1651. The switch current is detected by a sense resistor into

the CS pin. When the power switch is turned on, a turn-on spike will occur on this resistor. A 500ns leading-edge

blanking is built in to avoid false-termination of the switching pulse so that the external RC filtering is no longer

needed.