UC3869

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 8

www.unisonic.com.tw

QW-R103-110.c

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OPERATION DESCRIPTION

The UTC UC3869 devices integrate many useful designs into one controller for low-power switch-mode power

supplies. The following descriptions highlight some of the features of the UTC UC3800 series.

Internal High Voltage Startup and Under Voltage Lockout (UVLO)

to provide the operation current.

Operating Current

The typical operating current of UTC UC3869 is 0.8mA. Good efficiency is achieved with this low operating

current together with the ‘Extended burst mode’ control features.

Soft Start

UTC UC3869 features an internal 5ms soft start to soften the electrical stress occurring in the power supply

voltage is gradually increased from 0V to the maximum level. Every restart up begins with a soft start.

Frequency shuffling for EMI improvement

The frequency shuffling (switching frequency modulation) is implemented in UTC UC3869. The oscillation

frequency is modulated so that the tone energy is spread out. The spread spectrum minimizes the conduction band

EMI and therefore eases the system design.

Power-Saving Mode Operation

The proprietary Power-Saving Mode function provides linearly decreasing the switching frequency under

light-load conditions for higher efficiency. The feedback voltage, which is sampled from the voltage feedback loop, is

taken as the reference. Once the feedback voltage dropped below the threshold voltage, the switching frequency

starts to decrease. This Power-Saving Mode function dramatically reduces power consumption under light-load

conditions. The 22KHz minimum frequency control also eliminates the audio noise at any loading conditions.

At zero load condition, the magnitude of power loss is in proportion to the number of switching events within a

fixed period of time. Reducing switching events leads to the reduction on the power loss and thus conserves the

energy. The UTC UC3869 enter burst mode at standby condition to minimize the switching loss and reduces the

standby power consumption. Power supplies using the UTC UC3869 can easily meet even the strictest regulations

regarding standby power consumption.

Oscillator Operation

The switching frequency is internally fixed at 65KHZ. No external frequency setting components are required for

PCB design simplification.

Current Sensing and Leading Edge Blanking

Cycle-by-Cycle current limiting is offered in UTC UC3869 current mode PWM control. The switching current is

detected by a sense resistor connected to the CS pin. An internal leading edge blanking circuit chops off the sensed

voltage spikes due to snubber diode reverse recovery and surge gate current of power MOSFET at initial internal

power MOSFET on state. The current limiting comparator is disabled and cannot turn off the internal power

MOSFET during the blanking period. The PWM duty cycle is determined by the current sense input voltage and the

FB input voltage.

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 eliminates the sub-harmonic oscillation and thus reduces

the output ripple voltage.