CR6853
OPERATION DESCRIPTION
Current Mode
generate a 20uA constant current and a
65kHz PWM switching frequency. The
suggested operating frequency range of
CR6853 is within 50KHz to 150KHz.
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.
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
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 Mꢁ, 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 14V/8.8V. During
startup, the hold-up capacitor must be
charge to 14.0V through the startup resistor.
The hysteresis is implemented to prevent
the shutdown from the voltage dip during
startup.
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 0.5mA. The operation frequency of
oscillator is to descend gradually. When the
feedback current is over 0.55mA, the
frequency of oscillator is invariable, namely
22kHz.
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.
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 0.59mA, 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,
6500
FOSC
=
(kHz)
RI(KΩ)
For example, a 100kꢁ resistor RI could
Oct, 2008 V2.0
6/12
Chengdu Chip-Rail Tech. Co., Ltd.
http://www.chiprail.com
ETC [ ETC ]
