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OB2273 View Datasheet(PDF) - Unspecified

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OB2273 Datasheet PDF : 12 Pages
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OB2273
High Performance Current Mode PWM Controller
OPERATION DESCRIPTION
OB2273 is a highly integrated current mode PWM Vref_burst_L (the threshold enter burst mode) and
control IC optimized for high performance, low device enters Burst Mode control. The Gate drive
standby power and cost effective offline flyback output switches when FB input rises back to
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
Vref_burst_H (the threshold exit burst mode).
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
o The switching frequency is internally fixed at
t 65KHZ. No external frequency setting
reliable startup in application.
l Operating Current
The Operating current of OB2273 is low at 1.8mA.
a Good efficiency is achieved with OB2273 low
i operating current together with the ‘Extended
burst mode’ control features.
nt Soft Start
OB2273 features an internal 4ms soft start to
e soften the electrical stress occurring in the power
d supply during startup. It is activated during the
i power on sequence. As soon as VDD reaches
UVLO(OFF), the CS peak voltage is gradually
f increased from 0.05V to the maximum level.
n Every restart up is followed by a soft start.
o Frequency shuffling for EMI improvement
C The frequency Shuffling (switching frequency
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
modulation) is implemented in OB2273. The
oscillation frequency is modulated so that the tone
t energy is spread out. The spread spectrum
minimizes the conduction band EMI and therefore
h eases the system design.
g Extended Burst Mode Operation
i At light load or zero load condition, most of the
r power dissipation in a switching mode power
B supply is from switching loss on the MOSFET, the
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
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
nswitching frequency leads to the reduction on the
Opower loss and thus conserves the energy.
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
The switching frequency is internally adjusted at EMI system design is easier to achieve with this
no load or light load condition. The switch dedicated control scheme.
frequency reduces at light/no load condition to
improve the conversion efficiency. At light load or
no load condition, the FB input drops below Protection Controls
©On-Bright Electronics
Confidential
8
Preliminary Datasheet
OB_DOC_DS_227305
 

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