OB2358AP datasheet(7/9 Pages) ETC2

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OB2358

Current Mode PWM Power Switch

Freq Shuffling

OPERATION DESCRIPTION

The OB2358 is a low power off-line SMPS

Switcher optimized for off-line flyback converter

applications in sub 27W power range. 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 OB2358 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 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 OB2358 is low at 2mA.

Good efficiency is achieved with OB2358 low

operating current together with the ‘Extended burst

mode’ control features.

• Soft Start

OB2358 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 0.77V. Every restart

up is followed by a soft start.

• Frequency shuffling for EMI improvement

The frequency Shuffling (switching frequency

modulation) is implemented in OB2358.

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 of OB2358 is internally

fixed at 50KHZ. No external frequency setting

components

are

required

for

PCB

design

simplification.

• Current Sensing and Leading Edge Blanking

Cycle-by-Cycle current limiting is offered in

OB2358 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

MOSFET on 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 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 internal power MOSFET in OB2358 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.

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