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OPERATION DESCRIPTION
Current Mode
Compared to voltage mode control, current mode control has a current feedback loop. When the voltage of the Sense resistor
peak current of the primary winding reaches the internal setting value VTH, the register resets and the power MOSFET cuts off.
So, 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, and avoid the pole that the output filter
inductance brings and the two-class system descends to the one-class. So it widens the frequency range and optimizes
overload protection and short circuit protection.
Startup Current and Under Voltage Lockout
The startup current of RS2051 is set to be very low so that a large value startup resistor can be used to minimize the power loss.
For AC to DC adaptor with universal input range design, a 2 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 RS2051 is designed to 12.8V/7.8V. During startup, the hold-up capacitor must be
charge to 12.8V through the startup resistor. The hysteresis is implemented to prevent the shutdown from the voltage dip during
startup.
Internal Bias and OSC Operation
A resistor connected between RI pin and GND pin sets the internal constant current source to charge or discharge the internal
fixed capacitor. The charge time and discharge time determines the internal clock speed and the switching frequency.
Increasing the resistance will reduce the value of the input current and reduce the switching frequency. The relationship
between RI and PWM switching frequency follows the below equation within the RI allowed range.
6000
FOSC
=
(kHz)
RI(KΩ)
For example, a 100kΩ resistor RI could generate a 13uA constant current and a 60kHz PWM switching frequency.
The suggested operating frequency range of RS2051 is within 30KHz to 150KHz.
Green Power Operation
The power dissipation of switching mode power supply is very important in zero load or light load condition. The major
dissipation results from conduction loss、switching loss and consume of the control circuit. However, all of them relates to the
switching frequency. There are many difference topologies has been implemented in different chip. The basic operation theory
of all these approaches intends to reduce the switching frequency under light-load or no-load condition.
The RS2051`s green power function adapts PWM、PFM and CRM combining modulation. When RI resistor is 100kΩ, the
PWM frequency is 60kHz in medium or heavy load operation. Through modifying the pulse width, The RS2051 could control
output voltage. The current of FB pin increases when the load is in light condition and the internal mode controller enters
PFM&PWM when the feedback current is over 1.37mA. The operation frequency of oscillator is to descend gradually. When the
feedback current is over 1.43mA, the frequency of oscillator is invariable, namely 22kHz.
To decrease the standby consumption of the power supply, Chip-Rail introduces the Cycle Reset Mode technology (CRM). If the
feedback current is over 1.45mA, mode controller of the RS2051 would reset internal register all the time and cut off the GATE
pin. While the output voltage is lower than the set value, the register would be set, the GATE pin operate again. So the
frequency of the internal OSC is invariable, the register would reset some pulses so that the practical frequency is decreased at
the GATE pin.
RS2051 Green-Power Function
DS-RS2051-02
September, 2007
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
