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Fig.2. Auxiliary voltage waveform
Via a resistor divider connected between the auxiliary winding and INV (pin 3), the auxiliary voltage is sampled at the end of
the demagnetization and it is hold until the next sampling.
The sampled voltage is compared with VREF (2.0V) and the error is amplified. The error amplifier output COMP reflects the
load condition and controls the PWM switching frequency to regulate the output voltage, thus constant output voltage can
be achieved.
When sampled voltage is below VREF and the error amplifier output COMP reaches its maximum, the switching frequency is
controlled by the sampled voltage thus the output voltage to regulate the output current, thus the constant output current
can be achieved.
Adjustable CC point and Output Power
In RS2135, the CC point and maximum output power can be externally adjusted by external current sense resistor RS at CS pin
as illustrated in Typical Application Diagram. 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.3.
Fig.3. Adjustable output power by changing RS
Operation switching frequency
The switching frequency of RS2135 is adaptively controlled according to the load conditions and the operation modes. No
external frequency setting components are required. The operation switching frequency at maximum output power is set to
60KHz internally.
For flyback operating in DCM, The maximum output power is given by
1
POMAX = ×LP ×FSW ×IP2
2
Where LP indicate the inductance of primary winding and IP is the peak current of primary winding.
DS‐RS2135‐03 September, 2009
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