CM6800AGIP datasheet(12/18 Pages) CHAMP

12 / 18 page

CM6800A

LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO

2009/08/10 Rev. 1.6

Champion Microelectronic Corporation

Page 12

Oscillator (RAMP1)

The oscillator frequency is determined by the values of RT

and CT, which determine the ramp and off-time of the

oscillator output clock:

fOSC =

DEADTIME

RAMP

The dead time of the oscillator is derived from the following

equation:

tRAMP = CT x RT x In

3.75

1.25

REF

−

at VREF = 7.5V:

tRAMP = CT x RT x 0.51

The dead time of the oscillator may be determined using:

tDEADTIME =

2.65mA

2.5V

x CT = 943 x CT

The dead time is so small (tRAMP >> tDEADTIME ) that the

operating frequency can typically be approximately by:

fOSC =

RAMP

EXAMPLE:

For the application circuit shown in the datasheet, with the

oscillator running at:

fOSC = 67.5kHz =

RAMP

Solving for CT x RT yields 2.9 x 10

-5. Selecting standard

components values, CT = 470pF, and RT =61.9kΩ

The dead time of the oscillator adds to the Maximum PWM

Duty Cycle (it is an input to the Duty Cycle Limiter). With

zero oscillator dead time, the Maximum PWM Duty Cycle is

typically 45%. In many applications, care should be taken

that CT not be made so large as to extend the Maximum

Duty Cycle beyond 50%. This can be accomplished by

using a stable 390pF capacitor for CT.

PWM Section

Pulse Width Modulator

The PWM section of the CM6800A 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. The PWM is capable of current-mode or

voltage-mode operation. 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.

DCILIMIT, which provides cycle-by-cycle current limiting, is

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 VDC will be

compared. Under these conditions, the use of voltage

feedforward from the PFC buss can assist in line regulation

accuracy and response. As in current mode operation, the

DC ILIMIT input is used for output stage overcurrent protection.

No voltage error amplifier is included in the PWM stage of

the CM6800A, as this function is generally performed on the

output side of the PWM’s isolation boundary. To facilitate the

design of optocoupler feedback circuitry, an offset has been

built into the PWM’s RAMP2 input which allows VDC to

command a zero percent duty cycle for input voltages below

0.7V.

PWM Current Limit

The DC ILIMIT pin is a direct input to the cycle-by-cycle current

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 also softly discharge the voltage of

soft start capacitor. It will limit PWM duty cycle mode.

Therefore, the power dissipation will be reduced during the

dead short condition.

VIN OK Comparator

The VIN OK comparator monitors the DC output of the PFC

and inhibits the PWM if this voltage on VFB is less than its

nominal 2.25V. Once this voltage reaches 2.45V, which

corresponds to the PFC output capacitor being charged to its

rated boost voltage, the soft-start begins.

PWM Control (RAMP2)

When the PWM section is used in current mode, RAMP2 is

generally used as the sampling point for a voltage

representing the current on the primary of the PWM’s output

transformer, derived either by a current sensing resistor or a

current transformer. In voltage mode, it is the input for a

ramp voltage generated by a second set of timing

components (RRAMP2, CRAMP2),that will have a minimum value

of zero volts and should have a peak value of approximately

5V. In voltage mode operation, feedforward from the PFC

output buss is an excellent way to derive the timing ramp for

the PWM stage.

Soft Start

Start-up of the PWM is controlled by the selection of the

external capacitor at SS. A current source of 20

μ A supplies

the charging current for the capacitor, and start-up of the

PWM begins at around 0.7V. Start-up delay can be

programmed by the following equation:

CSS = tDELAY x

0.7V

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