DocID024474 Rev 1

17/35

L4984D

Application information

Figure 26. Line-modulated fixed-off-time modulator: a) internal block diagram; b) key

waveforms

With reference to the schematic and the relevant key waveforms in Figure 26, an off-time

proportional to the instantaneous line voltage is achieved by charging the capacitor CT with

a constant current ITIMER, accurately fixed internally and temperature compensated, while

the MOSFET is off and commanding MOSFET turn-on (and resetting CT at zero) as the

voltage across CT equals that on the MULT pin. The voltage on this pin is:

Equation 5

where KP is the divider ratio of the resistors biasing the MULT pin. As a result:

Equation 6

and the switching frequency is:

Equation 7

The timing capacitor CT, therefore, is selected with the following design formula:

Equation 8

within the multiplier linearity range (0 to 3 V) and ITIMER is specified in Section 3: Electrical

characteristics

.

AM13242v1

MULT

-

+

OFF

ON

0

TIMER

Q

S

R

+

-

PWM

Comparator

Driver

PWM Latch

GD

CS

COMP

Multiplier

GD

CS

S

R

Q

Multiplier output

MULT

TIMER

t

t

t

t

t

t

a)

b)

θ

=

sin

Vpk

K

V

P

MULT

P

TIMER

T

t

P

TIMER

T

OFF

K

I

C

K

sin

θ

Vpk

K

I

C

T

=

→

=

Vout

K

1

Vout

C

K

I

T

1

f

t

T

P

TIMER

sw

sw

=

=

=

sw

P

TIMER

T

f

Vout

K

I

C

=