U3848 Datasheet, PDF(5/12 Page) Unisonic Technologies – HIGH PERFORMANCE CURRENT MODE PWM CONTROLLERS

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U3848 Datasheet, PDF (5/12 Pages) Unisonic Technologies – HIGH PERFORMANCE CURRENT MODE PWM CONTROLLERS

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UC3848

LINEAR INTEGRATED CIRCUIT

Â FUNCTIONAL DESCRIPTION

The internal reference voltages and bias circuit work at VCC>12.6V, and shutdown at VCC< 8.1V.

(1) Soft-Start

When every IC power on, driver output duty cycle will be decided by voltage VSS on soft-start capacitor and VCS on

current sense resistor at beginning. After VSS reach 5.1V, the whole soft-start phase end, and driver duty cycle

depend on VFB and VCS. The relation among VSS, VFB and VOUT as followed FIG.3, here soft-start phase Tsoft-start

should more than VOUT start-up phase Tstart-up, otherwise, IC will enter false OLP protection state. Because after the

soft-start phase end, if VOUT remain in lower voltage, VFB more than 4.6V, then IC enter false OLP state.

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.

Finally soft-start also set OVP active phase. OVP active phase between VSS=0 and VSS=3.8V, OVP will not be

sensed after VSS reach 3.8V.The Soft-start phase TSSÄ

6 ms

TSS = 12 ms

120 ms

(CSS=0.05uF)

(CSS=0.1uF)

(CSS=1uF)

FIG.3 Soft-start phase

UNISONIC TECHNOLOGIES CO., LTD

www.unisonic.com.tw

5 of 12

QW-R103-033,C

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