OB2273MP datasheet(7/10 Pages) ETC2

7 / 10 page

OB2273

High Performance Current Mode PWM Controller

OPERATION DESCRIPTION

OB2273 is a highly integrated current mode PWM

control IC optimized for high performance, low

standby power and cost effective offline 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 OB2273 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.

•

Operating Current

The Operating current of OB2273 is low at 1.8mA.

Good efficiency is achieved with OB2273 low

operating current together with the ‘Extended

burst mode’ control features.

•

Soft Start

OB2273 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 CS peak voltage is gradually

increased from 0.15V to the maximum level.

Every restart up is followed by a soft start.

•

Frequency shuffling for EMI improvement

The frequency Shuffling (switching frequency

modulation) is implemented in OB2273.

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.

•

Extended Burst Mode Operation

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 transformer 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 drops below burst

mode threshold level and device enters Burst

Mode control. The Gate drive output switches only

when VDD voltage drops 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.

• Oscillator Operation

The switching frequency is internally fixed at

65KHZ.

external

frequency

setting

components are required for PCB design

simplification.

•

Current Sensing and Leading Edge Blanking

Cycle-by-Cycle current limiting is offered in

OB2273 current mode PWM control. The switch

current is detected by a sense resistor into the cs

pin. An internal leading edge blanking circuit

chops off the sensed voltage spike at initial

internal power MOSFET on state due to snubber

diode reverse recovery and surge gate current of

power MOSFET. 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 prevents the sub-

harmonic oscillation and thus reduces the output

ripple voltage.

•

Drive

The power MOSFET is driven by a dedicated gate

driver for power switch control. Too weak the gate

drive strength results in higher conduction and

switch loss of MOSFET while too strong gate

drive results the compromise of EMI.

A good tradeoff is achieved through the built-in

totem pole gate design with right output strength

and dead time control. The low idle loss and good

EMI system design is easier to achieve with this

dedicated control scheme.

•

Protection Controls

Confidential

Preliminary Datasheet

OB_DOC_DS_227302

OB2273MP Datasheet(PDF) 7 Page - List of Unclassifed Manufacturers

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