ZXLD1370

Document number: DS32165 Rev. 6 - 2

16 of 40

www.diodes.com

December 2016

© Diodes Incorporated

ZXLD1370

Therefore the average LED current is the coil current multiplied by the schottky diode duty cycle, 1-D.



































V

V

ADJ

_

GI

R

225

.

0

D

1

I

I

REF

ADJ

S

RS

LED

(Boost and Buck-Boost)

Equation 4

independent of the input and output voltages.

If the ADJ pin is connected to the REF pin, this simplifies to:

ADJ

_

GI

R

225

.

0

I

S

LED















(Boost and Buck-Boost)

Considering power dissipation and accuracy, it is useful to know how the mean sense voltage varies with input voltage and other parameters.































V

V

D

1

ADJ

_

GI

225

.

0

I

V

REF

ADJ

RS

RS

(Boost and Buck-Boost)

Equation 5

This shows that the sense voltage varies with duty cycle in Boost and Buck-Boost configurations.

Application Circuit Design

External component selection is driven by the characteristics of the load and the input supply, since this will determine the kind of topology being

used for the system. Component selection begins with the current setting procedure, the inductor/frequency setting and the MOSFET selection.

Finally after selecting the freewheeling diode and the output capacitor (if needed), the application section will cover the PWM dimming and

thermal feedback. The full procedure is greatly accelerated by the web Calculator spreadsheet, which includes fully automated component

selection, and is available on the Diodes website. However the full calculation is also given here.

Some components depend upon the switching frequency and the duty cycle. The switching frequency is regulated by the ZXLD1370 to a large

extent, depending upon conditions. This is discussed in a later paragraph dealing with coil selection.

Duty Cycle Calculation and Topology Selection

The duty cycle is a function of the input and output voltages. Approximately, the MOSFET switching duty cycle is:

V

V

D

IN

OUT

for Buck

V

V

V

D

OUT

IN

OUT

BOOST





for Boost

Equation 6

V

V

V

D

IN

OUT

OUT

BB





for Buck-Boost

Because D must always be a positive number less than 1, these equations show that:

for Buck (voltage step-down)

for Boost (voltage step-up)

for Buck-Boost (voltage step-down or step-up)

This allows us to select the topology for the required voltage range.