SC2569 datasheet(9/13 Pages) ETC

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SC2569

Safety-Chip

www.safety-chip.com

SC2569_Datasheet_V1.0_EN

Functional Description

The

SC2569

highly

integrated

PWM

controller

optimized

for

off-line

flyback

converter applications. The extended Burst Mode

control

greatly

reduces

the

standby

power

consumption and helps the design easily to meet

the

international

power

conservation

requirements.

Startup Current and Start up Control

Startup current of SC2569 is designed to be very

low so that VDD could be charged up above

VDD_ON and device starts up quickly. A large value

startup resistor can therefore be used to minimize

the power loss yet achieve a reliable startup in

application. For a typical AC/DC adaptor with

universal input range design, a 2M

Ω, 1/8W

startup resistor could be used together with a

VDD capacitor to provide a fast startup and yet

low power dissipation design solution.

Operating Current

The Operating current of SC2569 is low at 2.0mA

(typical).

Good

efficiency

achieved

with

SC2569 low operation current together with the

extended Burst Mode control features.

Soft Start

SC2569 features an internal 4ms (typical) soft

start to soften the electrical stress occurring in the

power supply during startup. It is activated during

the power on sequence. As soon as VDD reaches

VDD_ON, the SENSE peak voltage is gradually

increased from 0.1V to the maximum level. Every

restart up is followed by a soft start.

Frequency jitter for EMI improvement

The frequency jitter is implemented in SC2569.

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.

Oscillator Operation

The switching frequency of SC2569 is externally

programmable. A resistor connected between RI

and GND sets the constant current source to

charge/discharge the internal cap and thus the

PWM oscillator frequency is determined. The

relationship between RI and switching frequency

follows the below equation within the specified RI

in K

Ω range at nominal loading operational

condition.

(Khz)

KΩ

(

1560

OSC

）

Multi-mode Operation for High Efficiency

SC2569 is a multi-mode controller. The controller

changes the mode of operation according to the

FB pin voltage. At the normal operating condition,

the IC operates in （65KHz, if RI=24K

Ω）PWM

mode. As the output load current is decreased,

the IC enter into green mode smoothly from the

PWM

mode.

this

mode,

the

switching

frequency will start to linearly decrease from

65KHz to 22KHz. So the switching loss is

minimized and the high conversion efficiency can

be achieved. At light load or no load condition,

most of the power dissipation in a switching mode

power supply is from switching loss of the

MOSFET, the core loss of the transformer and the

loss of the snubber circuit. The magnitude of

power loss is in proportion to the switching

frequency. Lower switching frequency leads to

the reduction on the power loss and thus

conserves the energy. The switching frequency is

internally adjusted at no load or light load

condition. The switch frequency reduces at

light/no load condition to improve the conversion

efficiency.

At light load or no load condition, the FB input

drops below VRef_Burst_L and device enters Burst

Mode control. The Gate drive output switches

when FB input rises back to VRef_Burst_H. Otherwise

the gate drive remains at off state to minimize the

switching loss and reduces the standby power

consumption to the greatest extend.

Current Sensing and Leading Edge Blanking

Cycle-by-Cycle current limiting is offered in

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