U3848B Ver la hoja de datos (PDF) - Unisonic Technologies
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U3848B Datasheet PDF : 13 Pages
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U3848/A/B/C
LINEAR INTEGRATED CIRCUIT
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.
90
80
70
60
110V
50
40
220V
30
20
10
0 0.6 1.2 1.8 2.4 3 3.6 4.8 6 7.2 12
POUT(W)
(RT=75KΩ)
FIG.4 The relation curve between fSW and output power POUT
100
90
80
70
60
50
40
30
20
10
55 60 65 70 75 80 85 90 95
RT (kΩ)
( POUT=8W)
FIG.5 The relation curve between fSW and RT
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
8 of 13
QW-R103-033,B
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