Design equations for the modified buck converter

AN2928

8/21

2.2

Fixed off-time network calculation

The basic idea for this type of converter is to obtain a constant off-time when the power

MOSFET is turned off. This design approach is quite simple and cost-effective, because the

constant off-time is easily set by the RC network. Its calculation is described in this section.

The duty cycle is defined by the switching frequency and turn-off time:

Equation 9

f = switching frequency in CCM (Hz)

T = period in CCM (s)

D = duty cycle (-)

From Equation 8 and Equation 9, the turn off-time can be calculated (the switching

frequency is selected):

Equation 10

Equation 11

As stated above, the modified buck converter uses a FOT network. The off-time is set by

exponential law:

Equation 12

The voltage at the ZCD pin decreases until it reaches the internal triggering limit, which

causes switching to the turn-on stage. The trigger voltage for the L6562A is 0.7 V. The time

Equation 13

D

t

ON

T

---------

Tt

OFF

–

T

---------------------

1

t

OFF

T

-----------

1

=

–

ft

OFF

⋅

–

=

=

=

1f

t

OFF

V

LED

V

IN

--------------

=

⋅

–

t

OFF

1

V

LED

V

IN

--------------

–

⎝⎠

⎛⎞

f

-----------------------------

=

V

ZC D

V

ZCD CL AM P

e

1

R

4

C

4

⋅

---------------------

⎝⎠

⎛⎞

⋅

=

t

OFF

R

4

C

4

V

ZCD CL AM P

V

ZC D TRIGGER

-----------------------------------------

⎝⎠

⎜⎟

⎛⎞

R

4

C

4

5.7

0.7

--------

⎝⎠

⎛⎞

ln

R

4

C

4

2.1

⋅⋅

=

⋅⋅

=

ln

⋅⋅

=