US1652L-S08-R datasheet(4/9 Pages) UTC

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US1652

LINEAR INTEGRATED CIRCUIT

UNISONIC TECHNOLOGIES CO., LTD

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www.unisonic.com.tw

QW-R119-027.A

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FU N CT I ON AL DESCRI PT I ON

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 to 65kHz. 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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