ZXLD1370

Document number: DS32165 Rev. 6 - 2

17 of 40

www.diodes.com

December 2016

© Diodes Incorporated

ZXLD1370

More exact equations are used in the web Calculator. These are:





V

V

V

R

R

I

V

V

D

DSON

F

IN

COIL

S

OUT

F

OUT

BUCK













for Buck





V

V

V

V

R

R

I

V

V

D

DSON

F

OUT

F

COIL

S

IN

IN

OUT

BOOST















for Boost

Equation 7







V

V

V

V

R

R

I

I

V

V

D

DSON

F

IN

OUT

COIL

S

OUT

IN

F

OUT

BB

















for Buck-Boost

Where

= DC winding resistance of L1

The additional terms are relatively small, so the exact equations will only make a significant difference at lower operating voltages at the input

for Buck

=

for Boost

Equation 8

for Buck-Boost

estimated as follows.

or

So

V

9

.

0

V

N

I

I

IN

LED

LED

Equation 9

Equation 9

calculation of Duty Cycle and the selection of Buck, Boost or Buck-Boost topology.

An initial estimate of duty cycle is required before we can choose a coil. In Equation 7, the following approximations are recommended:

= 0.5V

= 0.5V

= 0.5V

= 0.1V

= 1.1V

Then Equation 7 becomes:

4

.

0

V

1

V

D

IN

OUT

BUCK







for Buck

4

.

0

V

1

V

V

D

OUT

IN

OUT

BOOST









for Boost

Equation 7a

4

.

0

V

V

6

.

1

V

D

IN

OUT

OUT

BB









for Buck-Boost