ML4802

10

Datasheet

August 2000

The major concern when compensating the ML4802's

voltage loop error amplifier is that the current amplifier

compensation is chosen to optimize frequency response

while maintaining good stability. This leads to the

following rules of thumb: the crossover frequency of the

current amplifier should be at least 10 times that of the

voltage amplifier to prevent interaction with the voltage

loop. It should also be limited to less than 1/6th that of

the switching frequency, e.g. 16.7kHz for a 100kHz

switching frequency.

For more information on compensating the current and

voltage control loops, see Application Notes 33, 34, and

55. Application Note 16 also contains valuable

information for the design of this class of PFC.

Oscillator

The oscillator frequency is determined by the values of RT

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

oscillator output clock:

The deadtime of the oscillator is derived from the

following equation:

at VREF = 7.5V:

The ramp of the oscillator may be determined using:

The deadtime is so small (tRAMP >> tDEADTIME) that the

operating frequency can typically be approximated by:

EXAMPLE:

For the application circuit shown in the data sheet, with

the oscillator running at:

Solving for RT x CT yields 1 x 10-4. Selecting standard

components values, CT = 100pF, and RT = 100k

W.

RAMP 1

The ramp voltage on this pin serves as a reference to

which the PFC control signal is compared in order to set

the duty cycle of the PFC switch. The external ramp

voltage is derived from an RC network similar to the

oscillator's. The PWM's oscillator sends a synchronous

pulse every other cycle to reset this ramp.

The ramp voltage should be limited to no more than the

output high voltage (6V) of the current error amplifier. The

timing resistor values should be selected such that the

capacitor will not charge past this point before being

reset. In order to ensure the linearity of the PFC loop's

transfer function and improve noise immunity, the

charging resistor should be connected to the 13.5V VCC

rather than the 7.5V reference. This will keep the charging

voltage across the timing capacitor in the "linear" region

of the charging curve.

The component value selection is similar to oscillator RC

component selection.

The charge time of RAMP 1 is derived from the following

equations:

At VCC = 13.5V and assuming RampPeak = 5V to allow

for component tolerances:

The capacitor value should remain small to keep the

discharge energy and the resulting discharge current

through the part small. A good value to use is the same

value used in the pwm timing circuit (CT).

For the application circuit shown in Figure 7, using a

200kHz PWM and a 100pF timing capacitor yeilds RT:

FUNCTIONAL DESCRIPTION (Continued)

fOSC =

1

tRAMP + DEADTIME

tRAMP = CT RT IN

VREF - 1.25

VREF - 3.75





tRAMP = CT RT 0.51

™™

tDEADTIME =

2.5V

5.5mA

CT = 455 CT

™™

fOSC =

1

tRAMP

fOSC = 200kHz =

1

tRAMP

tRAMP = 0.51 RT CT = 1 10

-5

™™

™

Rt

110

0.463

100 10

5

12

=

™

™™

-

-

27

Rt

215k

=W

tCHARGE = .463 R C

0

™™

tCHARGE = C R 1n

VCC - RampValley

VCC - RampPeak

™™









tCHARGE =

2

fOSC

fOSC =

1

tCHARGETIME + tDISCHARGETIME