OB2358AP View Datasheet(PDF) - On-Bright Electronics
Part Name
Description
Manufacturer
OB2358AP Datasheet PDF : 10 Pages
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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.
©On-Bright Electronics
-7-
Confidential
OB_DOC_DS_5800
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