CM6800T Champion Microelectronic Corp., CM6800T Datasheet - Page 19

• Manufacturer Part Number: CM6800T
• Description: Epa/85+ Pfc+pwm Combo Controller Design For High Efficient Power Supply
• Manufacturer: Champion Microelectronic Corp.

http://www.championmicro.com.tw
Oscillator (RAMP1, or called RTCT)
fpwm=68Khz and fpfc=68Khz, it provides the best
performance in the PC application.
The oscillator frequency, fRTCT is the similar formula in
CM6800:
following equation:
at VREF = 7.5V:
The dead time is so small (t
operating frequency can typically be approximately by:
Ct should be greater than 470pF.
standard components values, C
5.88kΩ
PWM Section
Pulse Width Modulator
The PWM section of the CM6800T is straightforward, but
there are several points which should be noted. Foremost
among these is its inherent synchronization to the PFC
section of the device, from which it also derives its basic
timing.
voltage-mode operation.
2010/08/03
In CM6800T, fRTCT=4xfpwm=4xfpfc fRTCT=272Khz,
The dead time of the oscillator is derived from the
The dead time of the oscillator may be determined using:
Let us use 1000PF Solving for R
The dead time of the oscillator determined two things:
1.) PFC minimum off time which is the dead time
2.) PWM skipping reference duty cycle: when the PWM
fRTCT =
t
t
t
fRTCT =
RAMP
RAMP
DEADTIME
duty cycle is less than the dead time, the next cycle
will be skipped and it reduces no load consumption
in some applications.
The
= C
= C
Rev. 1.2
=
T
T
PWM
t
t
x R
x R
RAMP
RAMP
3.64mA
1
2.5V
T
T
x In
x 0.51
+
is
1
t
DEADTIME
x C
V
V
capable
REF
REF
T
RAMP
= 686.8 x C
−
−
1.25
3.75
T
T
>> t
= 1000pF, and R
yields 5.88K. Selecting
of
DEADTIME
current-mode
Champion Microelectronic Corporation
T
) that the
T
or
=
In current-mode applications, the PWM ramp (RAMP2) is usually
derived directly from a current sensing resistor or current
transformer in the primary of the output stage, and is thereby
representative of the current flowing in the converter’s output
stage. DCI
typically connected to RAMP2 in such applications. For
voltage-mode, operation or certain specialized applications,
RAMP2 can be connected to a separate RC timing network to
generate a voltage ramp against which V
Under these conditions, the use of voltage feed-forward from the
PFC buss can assist in line regulation accuracy and response. As
in current mode operation, the DC I
stage over-current protection.
CM6800T, as this function is generally performed on the output
side of the PWM’s isolation boundary. To facilitate the design of
opto-coupler feedback circuitry, an offset has been built into the
PWM’s RAMP2 input which allows V
percent duty cycle for input voltages below around 1.8V.
PWM Current Limit (DCILIMIT)
limiter for the PWM section. Should the input voltage at this pin
ever exceed 1V, the output flip-flop is reset by the clock pulse at
the start of the next PWM power cycle. Beside, the cycle-by-cycle
current, when the DC ILIMIT triggered the cycle-by-cycle current.
It will limit PWM duty cycle mode. Therefore, the power
dissipation will be reduced during the dead short condition.
CM6800T’s PWM section becomes a current mode PWM
controller. Sometimes, network between DCILIMIT and RAMP2 is
a resistor divider so the DCILIMIT’s 1V threshold can be amplified
to 1.8V or higher for easy layout purpose.
PWM Brown Out (380V-OK Comparator)
and inhibits the PWM if this voltage on V
2.36V. Once this voltage reaches 2.36V, which corresponds to
the PFC output capacitor being charged to its rated boost voltage,
the soft-start begins. It is a hysteresis comparator and its lower
threshold is 1.35V.
EPA/85+ PFC+PWM COMBO CONTROLLER
The DC I
When DCILIMIT pin is connected with RAMP2 pin, the
The 380V-OK comparator monitors the DC output of the PFC
No voltage error amplifier is included in the PWM stage of the
CM6800T
Design for High Efficient Power Supply
LIMIT
LIMIT
, which provides cycle-by-cycle current limiting, is
pin is a direct input to the cycle-by-cycle current
(Turbo-Speed PFC+Green PWM)
LIMIT
DC
FB
input is used for output
is less than its nominal
DC
to command a zero
will be compared.
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