OB2269AP datasheet(8/12 Pages) LITEON

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©On-Bright Electronics

Confidential

A Liteon Semiconductor Company

- 8 -

V2.1

OB2268

OB2269

Current Mode PWM Controller

OPERATION DESCRIPTION

The OB2268/9 is a highly integrated PWM

controller

optimized

for

offline

flyback

converter applications in above 20W 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 OB2268/9 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 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 yet low power dissipation

design solution.

• Operating Current

The Operating current of OB2268/9 is low at

2.3mA.

Good

efficiency

achieved

with

OB2268/9 low operating current together with

extended burst mode control schemes.

• Frequency shuffling for EMI improvement

The

frequency

Shuffling/jittering

(switching

frequency modulation) is implemented in OB2269.

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 eases the system design.

• Burst Mode Operation

At zero load or light load condition, most 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.

OB2268/9

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 (1.8V). 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 thus reduce the

standby power consumption to the greatest extend.

The nature of high frequency switching also

reduces 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.

FPWM = 6500/RI (KHZ)

• Current Sensing and Leading Edge Blanking

Cycle-by-Cycle current limiting is offered in

OB2268/9 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

needed. The current limit comparator is disabled

and cannot turn off the external 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.

• Over Temperature Protection

An NTC resistor in series with a regular resistor

should

connect

between

and

GND

for

temperature sensing and protection. NTC resistor

value becomes lower when the ambient temperature

rises. With the fixed internal current IRT flowing

through the resistors, the voltage at RT pin

becomes lower at high temperature. The internal

OTP

circuit

triggered

and

shutdown

the

MOSFET when the sensed input voltage is lower

than VTH_OTP.

• Gate Drive

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