U3848-S08-R datasheet(8/13 Pages) UTC

8 / 13 page

U3848/A/B/C

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

8 of 13

www.unisonic.com.tw

QW-R103-033,B

FUNCTIONAL DESCRIPTION(Cont.)

(2) Switch Frequency Set

The maximum switch frequency decided by an external resistor RT connected between pin RT and ground. Then

the maximum switch frequency will depend on user requirement. The relation curve between fSW and RT as followed

FIG.5 under the condition of POUT=8W. The equation between fSW and RT as followed (1-2) for U3848A/C, (1-3) for

U3848/B:

fSW = -0.61*RT+115.7--------------------------(1-2)

fSW = -1.1*RT+167.5--------------------------(1-3)

After RT connected, switch frequency is also modulated by output power POUT during IC operating. So lower switch

frequency at lower load, which more and more improve IC’s efficiency at light load. Switch frequency is decreased

minimum at no load, then the U3848/B will operate at Power-Saving mode for Lower standby power. The relation

curve between fSW and POUT as followed FIG.4 under the condition of RT=75kΩ. The frequency wave of U3848A/C is

similar with U3848/B’s, and for them, maximum frequency point is 70kHz.

0.6 1.2 1.8 2.4 3 3.6 4.8 6

7.2 12

POUT(W)

(RT=75KΩ)

220V

110V

FIG.4 The relation curve between fSW and output power POUT

RT (kΩ)

(POUT=8W)

100

FIG.5 The relation curve between fSW and RT

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