OB2269C
Current Mode PWM Controller
Frequency Shuffling
OPERATION DESCRIPTION
The OB2269C is a highly integrated PWM
controller IC optimized for offline flyback
converter applications. 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 OB2269C is designed to be very
low so that VDD could be charged up above
UVLO(exit) 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 a typical 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 OB2269C is low at
2.3mA. Good efficiency is achieved with OB2269C
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
OB2269C. The oscillation frequency is modulated
with a internally generated random source so that
the tone energy is evenly spread out. The spread
spectrum minimizes the conduction band EMI and
therefore eases the system design in meeting
stringent EMI requirement.
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.
F
OSC
=
nf
id
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Current Sensing and Leading Edge
Blanking
Cycle-by-Cycle current limiting is offered in
OB2269C 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
required. The current limit comparator is disabled
and thus cannot turn off the external MOSFET
during the blanking period. 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.
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.
OB2269C 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
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ig
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Co
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On
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Over Temperature Protection
A 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 I
RT
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 V
TH
_OTP.
Gate Drive
Confidential
OB_DOC_DS_69CC1
Ly
co
n
1560
(
Khz
)
RI
(
Kohm
)
(志
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.
恒
通
电
子
)
ETC [ ETC ]
