CR6853 Просмотр технического описания (PDF) - ChengduChip-RailTech

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CR6853

Novel Low Cost Green-Power PWM Controller

ChengduChip-RailTech 

CR6853

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 CR6853 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

CR6853 is designed to 12.8V/7.8V. During

startup, the hold-up capacitor must be

charge to 13.0V 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.

FOSC

=

6500 (kHz)

RI (KΩ)

For example, a 100kΩ resistor RI could

generate a 13uA constant current and a

65kHz PWM switching frequency. The

suggested operating frequency range of

CR6853 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 CR6853`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 CR6853 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 1.0mA. The operation frequency of

oscillator is to descend gradually. When the

feedback current is over 1.40mA, the

frequency of oscillator is invariable, namely

22kHz.

CR6853 Green-Power Function

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 CR6853 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,

Dec, 2007 V1.1

Chengdu Chip-Rail Tech. Co., Ltd.

6/11

http://www.chiprail.com

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