UC3873

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

6 of 10

www.unisonic.com.tw

QW-R103-068. D

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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 gate driver.

Constant Output Power Limit

than that under low input line voltage. To compensate this variation for wide AC input range, the threshold voltage is

current is increased. Smaller threshold voltage, forces the output GATE drive to terminate earlier, thus reduce the

total PWM turn-on time and make the output power equal to that of low line input. This proprietary internal

compensation ensures a constant output power limit for wide AC input voltage from 90VAC to 264VAC.

Under Voltage Lockout (UVLO)

Gate Output

The UC3873 output stage is a fast totem pole gate driver. Cross conduction has been avoided to minimize heat

dissipation, increase efficiency, and enhance reliability. A good tradeoff is achieved through dead time control. The

low idle loss and good EMI system design is easier to achieve with this dedicated control scheme. An internal 15V