US3702 datasheet(7/9 Pages) UTC

7 / 9 page

US3702

Preliminary

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

7 of 9

www.unisonic.com.tw

QW-R119-022.c



FUNCTIONAL DESCRIPTION(Cont.)

(3) Internal Synchronized 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.

(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 primary current, across the primary wind of transfer, reaches the limit current, around 1.2A, 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 for this output power limit variation across a wide AC input

range, the limit current in primary winding is adjusted by adding a positive ramp. This ramp signal rises from 1.14A to

1.2A, and then flattens out at 1.2A. A smaller limit current forces the output GATE drive to terminate earlier. This

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

internal compensation ensures a constant output power limit for a wide AC input voltage range (90VAC~264VAC).

(6) 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.

OLP

After power on, IC will shutdown driver if over load state occurs for continual TD-OLP. OLP case as followed Fig. 6.

The test circuit as followed Fig.8

OVP

OVP will shutdown the switching of the power MOSFET whenever VCC>VOVP. The OVP case as followed Fig.7.

the test circuit as followed Fig.9.

US3702 Datasheet(PDF) 7 Page - Unisonic Technologies