UC3848L-S08-R datasheet(7/11 Pages) ETL

7 / 11 page

UC3848

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

7 of 11

www.unisonic.com.tw

QW-R103-033.F

FUNCTIONAL DESCRIPTION(Cont.)

(4) 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.5. 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.5 Frequency Hopping

(5) Constant Output Power Limit

When the SENSE voltage, across the sense resistor RS, reaches the threshold voltage, around 0.8V, the output

GATE drive will be turned off after a small propagation delay tD. This propagation delay will introduce an additional

current proportional to tD×VIN/Lp. Since the propagation delay is nearly constant regardless of the input line voltage

VIN. Higher input line voltage will result in a larger additional current and hence the output power limit is also higher

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

adjusted by the VIN current. Since VIN pin is connected to the rectified input line voltage through a resistor RVIN, a

higher line voltage will generate higher VIN current into the VIN pin. The threshold voltage is decreased if the VIN

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.

(6) Driver Output Section

The driver-stage drives the gate of the MOSFET and is optimized to minimize EMI and to provide high circuit

efficiency. This is done by reducing the switch on slope when reaching the MOSFET threshold. This is achieved by a

slope control of the rising edge at the driver’s output. The output driver is clamped by an internal 15V Zener diode in

order to protect power MOSFET transistors against undesirable gate over voltage.

(7) Protection section

The IC takes on more protection functions such as OLP, OVP and OTP etc. In case of those failure modes for

continual blanking time, the driver is shut down. At the same time, IC enters auto-restart, VCC power on and driver is

reset after VCC power on again.

OTP

OTP will shut down driver when junction temperature TJ>T (THR) for continual a blanking time.

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