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OPERATION DESCRIPTION
Current Model
Compared to voltage model control, current model control has a current feedback loop. When the voltage of the
sample resistor peak current of the primary winding reaches the internal setting value V
TH
, comparator reverse,
register reset and power BJT or MOSFET cut-off. So that to detect and modulate the peak current cycle by cycle
could control the output of the power supply. The current feedback has a good linear modulation rate and a fast input
and output dynamic impact avoid the pole that the output filter inductance brings and the second class system
descends to first class and so it widens the frequency range and optimizes overload protection and short circuit
protection.
Startup Current and Under Voltage Lockout
The startup current of RS2030 is set to be very low (3uA) so that a large value startup resistor can therefore be used
to minimize the power loss. For AC to DC adaptor with universal input range design, a 1.5 MΩ, 1/8 W startup resistor
and a 10uF/25V VDD hold capacitor could be used.
The turn-on and turn-off threshold of the RS2030 is designed to 5.4V/3.6V. During startup, the hold-up capacitor must
be charge to 5.4V through the startup resistor. The hysteresis is implemented to prevent the shutdown from the
voltage dip during startup.
Green Power Operation
The power dissipation of switching mode power supply is very important in zero load or light load condition. The major
dissipation result from conduction loss、switching loss and consume of the control circuit. However, all of them related
to the switching frequency. There are many difference topologies has been implemented in different chip. The basic
operation theory of all these approaches intended to reduce the switching frequency under light-load or no-load
condition.
RS2030’s green power function adapts PWM、PFM and CRM combining modulation. The PWM frequency is
65kHz/100kHz in medium or heavy load operation. Through modifying the pulse width, RS2030 could control output
voltage. To decrease the standby consumption of the power supply , Chip-Rail introduces the Cycle Reset Mode
technology；If the feedback current were over 1.5mA, mode controller of RS2030 would reset internal register all the
time and cut off the power BJT, while the output voltage is lower than the set value, it would set register, SW pin
operating again. Although the frequency of the internal OSC is invariable, the register would reset some pulses so
that the practical frequency is decreased at the SW pin.
RS2030 Green-Power Function
Internal Synchronized Slop Compensation
Although there are more advantages of the current mode control than conventional voltage mode control, there are
still several drawbacks of peak-sensing current-mode converter. Especially the open loop instability when it operates
in higher than 50% of the duty-cycle. RS2030 is introduced an internal slope compensation adding voltage ramp to
the current sense input voltage for PWM generation to solve this problem. It improves the close loop stability greatly
at CCM, prevents the sub-harmonic oscillation and thus reduces the output ripple voltage.
DS-RS2030X-EN-V1 Dec, 2007
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
