US1651 Datasheet, PDF (4/7 Pages) Unisonic Technologies – PRIMARY-SIDE REGULATION PWM POWER SWITCH

US1651
Preliminary
CMOS IC
 OPERATION DESCRIPTION
The UTC US1651 is a primary control switch mode charger and adapter applications. It operates in primary-side
sensing and regulation. Opto-coupler and TL431 could be eliminated. Proprietary built-in CV and CC control can
achievegood CV/CC performance. The device integrates PWM controller to enhance the performance of
discontinuous conduction mode (DCM) flyback converters.
Startup Current and Start up Control
The V_DD pin of UTC US1651 is connected to the line input through a resistor. A large value startup resistor can be
used to minimize the power loss in application because the start current of UTC US1651 is very low. When the V_DD
voltage reaches V_TH(ON), the internal startup circuit is disabled and the IC turns on.
Operating Current
The Operating current of UTC US1651 is as low as 1mA. Good efficiency and very low standby power can be
achieved.
Soft Start Control
The UTC US1651 features an internal 4ms soft start to minimize the component electrical over-stress during
power on startup. As soon as V_DD reaches UVLO (OFF), the control algorithm will ramp peak current voltage
threshold gradually from nearly zero to normal setting of 0.82V. Every restart is a soft start.
Constant Voltage Operation
The UTC US1651 is designed to produce good CC/CV control characteristic as shown in the Fig. 1.
To support UTC US1651 proprietary CC/CV control, system needs to be designed in DCM mode for flyback
system. When INV is equivalent or larger than 1.25V, the system works in CV control, In CV operation, the output
voltage is sensed on the primary side and the sensed signal controls the duty cycle through a built-in error amplifier
(EA). Connected to a resistor divider from the primary side sensing winding, the inverting input of the Error Amplifier
(EA) is compared to an internal reference voltage of 1.25V to regulate the output voltage. The EA output is internally
connected to the PWM generator and controls the duty cycle.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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QW-R129-005.a