CRE6559 FREQ SHUFFLING

CRE6559 FREQ SHUFFLING

The CRE6559 is a low power off-line PWM Switcher

optimized

for

o f f -line

fly

back

c onverter

applications in sub 30W power range. The

”Extended burst mode” control greatly reduces the

standby power consumption and help the design

easily to meet the international power conservation

requirements.

Startup current of CRE6559 is designed to be very

low so that VDD could be charged up above UVLO

threshold level 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 AC/DC adaptor with

universal input range design, a 2M , 1/8Wstartup

resistor could be used together with a VDD

capacitor to provide a fast startup and yet low

power dissipation design solution.

Ω

The Operating current of CRE6559 is low at

2mA.Good efficiency is achieved with CRE6559low

operating current together with the “Extended burst

mode” control features.

CRE6359 features an internal 4ms 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

UVLO(OFF), the peak current is gradually increased

from nearly zero to the maximum level of 0.83V,

Every restart up is followed by a soft start.

The frequency Shuffling (switching frequency

modulation) is implemented in CRE6559. 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.

At light load or zero load condition, most of the

power dissipation in a switching mode power

supply is from switching loss on the mosfet, the

core loss of the ransformer and the loss on 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 drop

below burst mode threshold level and device

enters Burst Mode control.

The Gate drive output switches only when VDD

voltage drop below a preset level and FB input is

active to output an on state. Otherwise the gate

drive remains at off state to minimize the switching

loss and reduces the standby power consumption

to the greatest extend.The switching frequency

control also eliminates the audio noise at any

loading conditions.

The switching frequency of CRE6559 is internally

fixed at 65KHZ. No external frequency setting

components a re required for P CB design

simplification

Cycle-by-Cycle c urrent limiting is offered i n

CRE6559 current mode PWM control. The switch

current is detected by a sense resistor into the sense

pin. An internal leading edge blanking circuit chops

off the sensed voltage spike at initial internal power

MOSFETon state due to snubber diode reverse

recovery and surge gate current of internal power

MOSFET so that the external RC filtering on sense

input is no longer needed. The current li comparator

is disabled and cannot turn off the blanking period.

The PWM duty cycle is determined by the current

sense input voltage and the FB input

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