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DS-RS2051-09 No v, 2010

www.Orister.com

OPERATION DESCRIPTION

Current Mode

Compared to voltage mode control, current mode control has a current feedback loop. When the voltage of the Sense resistor

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 14V/8.8V. During startup, the hold-up capacitor must be charge

to 14V 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.

)

kHz

(

)

K

(

RI

6500

Ω

=

For example, a 100kΩ resistor RI could

generate a 20uA constant current and a 65KHz PWM switching frequency.

The suggested operating frequency range of RS2051 is within 50KHz 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 65KHz 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 0.5mA, the frequency of oscillator is invariable, namely 22kHz.

To decrease the standby consumption of the power supply,

Orister introduces the Cycle Reset Mode technology (CRM). If the

feedback current is over 0.59mA, 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