www.DataSheet4U.com
CR6848
OPERATION DESCRIPTION
Current Model
suggested operating frequency range of
CR6848 is within 48KHz to 100KHz.
Compared to voltage model control,
Green Power Operation
current model control has
a
current
feedback loop. When the voltage of the
sense resistor peak current of the primary
winding reaches the internal setting value
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.
VTH, comparator reverse, register reset and
power 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.
CR6848`s green power function adapts
PWM、PFM and CRM combining modulation.
When RI resistor is 100k, the PWM
frequency is 58kHz in medium or heavy load
operation. Through modifying the pulse
width, CR6848 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 0.92mA. The
operation frequency of oscillator is to
descend gradually. The invariable frequency
of oscillator is 11.6kHz when the feedback
current is over 1.05mA. To decrease the
standby consumption of the power supply ,
Chip-Rail introduces the Cycle Reset Mode
technology；If the feedback current were
over 1.1mA, mode controller of CR6848
would reset internal register all the time and
cut off the gate pin, while the output voltage
is lower than the set value, it would set
register, gate 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 gate pin.
Startup Current and Under Voltage
Lockout
The startup current of CR6848 is set to be
very low 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
CR6848 is designed to 16.1V/11.1V. During
startup, the hold-up capacitor must be
charge to 16.1V 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 set the internal constant current
source to charge or discharge the internal
fixed cap. The charge time and discharge
time determine 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 pin and PWM
switching frequency follows the below
equation within the RI allowed range.
5800
FOSC
=
(kHz)
RI(KΩ)
For example, a 100kΩ resistor RI could
generate a 50uA constant current and a
58kHz PWM switching frequency. The
CR6848 Green-Power Function
Dec, 2006
V1.5
8/14
Chengdu Chip-Rail Tech.Co.Ltd.
http://www.chiprail.com
ETC [ ETC ]
