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UPS3702 Datasheet(PDF) 5 Page - Unisonic Technologies

Part No. UPS3702
Description  HIGH PERFORMANCE CURRENT MODE POWER SWITCH WITH ZERO CURRENT DETECTION
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Maker  UTC [Unisonic Technologies]
Homepage  http://www.utc-ic.com
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UPS3702 Datasheet(HTML) 5 Page - Unisonic Technologies

   
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UPS3702
Preliminary
LINEAR INTEGRATED CIRCUIT
UNISONICTECHNOLOGIESCO.,LTD
5 of 9
www.unisonic.com.tw
QW-R119-022.b
FUNCTIONAL DESCRIPTION
The internal reference voltages and bias circuit work at VCC> VTHD (ON), and shutdown at VCC<VCC (MIN).
(1) Soft-Start
When every IC power on, driver output duty cycle will be decided by inter-slope voltage VSOFTS and VCS on
current sense resistor at beginning. After the whole soft-start phase end, and driver duty cycle depend on VFB and
VCS. The relation among VSOFTS, VFB and VOUT as followed Fig.3. Furthermore, soft-start phase should end before VCC
reach VCC (MIN) during VCC power on. Otherwise, if soft-start phase remain not end before VCC reach VCC (MIN) during
VCC power on, IC will enter auto-restart phase and not set up VOUT.
Fig.3 Soft-start phase
(2) Switching Frequency Set
The maximum switching frequency is set to75kHz. Switching frequency is modulated by output power POUT
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 snubber
circuit. The magnitude of power loss is in proportion to the number of switching events within a fixed period of time.
So lower Switching 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 fSW and POUT/POUT (MAX) as followed Fig.4.
Fig. 4 The relation curve between fSW and relative output power POUT/ POUT (MAX)


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