UC3846

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

6 of 10

www.unisonic.com.tw

QW-R103-053.B

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 adds voltage ramp onto the current sense input voltage for PWM generation,

this greatly improves the close loop stability at CCM and prevents the sub-harmonic oscillation and thus reduces the

output ripple voltage.

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)

The turn-on and turn-off thresholds of the UTC UC3846 are fixed internally at 14.2V/8.2V. During start-up, the

hold-up capacitor must be charged to 14.2V through the start-up resistor, so that the UTC UC3846 will be enabled.

Gate Output

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