OB2263 datasheet(9/13 Pages) ETC2

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OB2263

Current Mode PWM Controller

Frequency Shuffling

OPERATION DESCRIPTION

The OB2263 is a highly integrated PWM controller

IC optimized for offline flyback converter

applications in sub 30W power range. The extended

burst mode control greatly reduces the standby

power consumption and helps the design easily

meet

the

international

power

conservation

requirements.

• Startup Current and Start up Control

Startup current of OB2263 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 provides reliable

startup in application. For AC/DC adaptor with

universal input range design, a 2 MΩ, 1/8 W

startup resistor could be used together with a VDD

capacitor to provide a fast startup and low power

dissipation solution.

• Operating Current

The Operating current of OB2263 is low at 1.4mA.

Good efficiency is achieved with OB2263 low

operating current together with extended burst

mode control features.

• Frequency shuffling for EMI improvement

The

frequency

Shuffling/jittering

(switching

frequency modulation) is implemented in OB2263.

The oscillation frequency is modulated with a

random source so that the tone energy is spread out.

The spread spectrum minimizes the conduction

band EMI and therefore reduces system design

challenge.

• Extended Burst Mode Operation

At zero load or light load condition, majority of the

power dissipation in a switching mode power

supply is from switching loss on the MOSFET

transistor, the core loss of the transformer and the

loss on the snubber circuit. 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.

OB2263 self adjusts the switching mode according

to the loading condition. At from no load to

light/medium load condition, the FB input drops

below burst mode threshold level. 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 frequency control also eliminates the audio

noise at any loading conditions.

• Oscillator Operation

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 Kohm range at nominal

loading operational condition.

)

(

)

(

6500

Khz

Kohm

OSC =

• Current Sensing and Leading Edge Blanking

Cycle-by-Cycle current limiting is offered in

OB2263 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 sense voltage spike at initial MOSFET on

state due to Snubber diode reverse recovery so that

the external RC filtering on sense input is no longer

required. The current limit comparator is disabled

and thus cannot turn off the external MOSFET

during the blanking period. 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.

• Gate Drive

OB2263 Gate is connected to an external MOSFET

gate for power switch control. Too weak the gate

drive strength results in higher conduction and

switch loss of MOSFET while too strong gate drive

output compromises the 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. An internal 18V clamp is

added for MOSFET gate protection at higher than

expected VDD input.

Confidential to Micro Bridge

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