US1651 Datasheet, PDF (5/7 Pages) Unisonic Technologies – PRIMARY-SIDE REGULATION PWM POWER SWITCH

US1651
Preliminary
CMOS IC
 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.
V_O
Large R_S
Small R_S
I_O
Fig 2. Adjustable output power by changing R_S
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.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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QW-R129-005.a