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OB2269 View Datasheet(PDF) - LiteOn Technology

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OB2269
LiteOn
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OB2269 Datasheet PDF : 0 Pages
OB2268
OB2269
Current Mode PWM Controller
OPERATION DESCRIPTION
The OB2268/9 is a highly integrated PWM
controller IC 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 M, 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 is 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 RT 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 is triggered and shutdown the
MOSFET when the sensed input voltage is lower
than VTH_OTP.
Gate Drive
©On-Bright Electronics
A Liteon Semiconductor Company
-8-
Confidential
V2.1
 

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