Primary-Side Regulation PWM Power Switch
OB2211 and OB2212 are cost effective PWM
power switch optimized for off-line low power
AC/DC applications including battery chargers and
adaptors. It operates in primary side sensing and
regulation, thus opto-coupler and TL431 are not
required. Proprietary CC control and built-in error
amplifier can achieve a good CC/CV performance.
z Startup Current and Start up Control
Startup current of OB2211/2 is designed to be very
low so that VDD could be charged up above UVLO
threshold and starts up quickly. A large value
startup resistor can therefore be used to minimize
the power loss in application.
z Operating Current
The Operating current of OB2211/2 is as low as
1mA. Good efficiency is achieved with the low
operating current together with ‘Extended burst
mode’ control features.
z Soft Start
OB2211/2 features an internal 4ms soft start to
minimize the component electrical over-stress
during power on startup. As soon as VDD reaches
UVLO(OFF), the control algorithm will ramp peak
current voltage threshold gradually from nearly
zero to normal setting of 0.82V. Every restart is a
z CC/CV Operation
OB2211/2 is designed to produce good CC/CV
control characteristic as shown in the Fig. 1.
In charger applications, a discharged battery
charging starts in the CC portion of the curve until
it is nearly full charged and smoothly switches to
operate in CV portion of the curve.
In an AC/DC adapter, the normal operation occurs
only on the CV portion of the curve. The CC
portion provides added cycle-by-cycle current
limiting protection. In CV operation, the output
voltage is sensed on the primary side and the
sensed signal controls the duty cycle through a
built-in error amplifier (EA).
To support OB2211/2 proprietary CC/CV control,
system needs to be designed in DCM mode for
flyback system. When INV is equivalent or larger
than 1.25V, the system works in CV control,
Otherwise, the system works in CC control.
z Error Amplifier (EA)
Connected to a resistor divider from the primary
side sensing winding, the inverting input of the
Error Amplifier (EA) is compared to an internal
reference voltage of 1.25V to regulate the output
voltage. The EA output is internally connected to
the PWM generator and controls the duty cycle.
z Adjustable CC point and Output Power
In OB2211/2, the CC point and output power
can be externally adjusted by external current
sense resistor Rs at CS pin. The output power
is adjusted through CC point change. The
larger Rs, the smaller CC point is, and the
smaller output power becomes, and vice versa
as shown in Fig.2.
Fig. 2 Adjustable output power by changing Rs
z 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, 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.
OB2211/2 self adjusts the switching frequency
according to the loading condition. The switch
frequency is reduced at light/no load condition to
improve the conversion efficiency. At light load/no
load condition, the output of the Error amplifier
(EA) drops below the burst mode threshold level
and device enters Burst Mode control. The
frequency control also eliminates the audio noise at
any loading conditions.
z Operation switching frequency
The switching frequency of OB2211/2 is internally
set and adjusted according to CC/CV operation. No
external frequency setting components are required.
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