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PDF US3706 Datasheet ( Hoja de datos )

Número de pieza US3706
Descripción CURRENT MODE PWM POWER SWITCH
Fabricantes Unisonic Technologies 
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US3706 datasheet

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US3706 pdf
US3706
Preliminary
CMOS IC
OPERATION DESCRIPTION (Cont.)
Frequency Hopping For EMI Improvement
The Frequency hopping is implemented in the IC; there are two oscillators built-in the IC. The first oscillator is to
set the normal switching frequency; the switching frequency is modulated with a period signal generated by the 2nd
oscillator. The relation between the first oscillator and the 2nd oscillator as followed Fig.2. So the tone energy is
evenly spread out, the spread spectrum minimizes the conduction band EMI and therefore eases the system design
in meeting stringent EMI requirement.
Fig.2 Frequency Hopping
Built-in Slope Compensation
Built-in slope compensation circuit greatly improves the close loop stability at CCM and prevents the
sub-harmonic oscillation.
Leading-Edge Blanking
Each time the power MOSFET is switched on, a turn-on spike will inevitably occur at the sense-resistor. To avoid
premature termination of the switching pulse, a 400ns leading-edge blanking time is built in. Conventional RC
filtering can therefore be omitted. During this blanking period, the current-limit comparator is disabled and it cannot
switch off the power MOSFET.
Under Voltage Lockout (UVLO)
The turn-on and turn-off thresholds of the US3706 are fixed internally at VDD(ON)/VDD(MIN) During start-up, the
hold-up capacitor must be charged to VDD(ON) through the start-up resistor, so that the US3706 will be enabled. The
hold-up capacitor will continue to supply VDD until power can be delivered from the auxiliary winding of the main
transformer. VDD must not drop below VDD(MIN) during this start-up process. This UVLO hysteresis window ensures
that hold-up capacitor will be adequate to supply VDD during start-up.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
5 of 8
QW-R129-001.b

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