ZXLD1371

ZXLD1371

Document number: DS35436 Rev. 1 - 2

23 of 42

www.diodes.com

February 2012

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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 web site. However the full calculation is also given here.

Please note the following particular feature of the web Calculator. The GI ratio can be set for Automatic calculation, or it can

be fixed at a chosen value. When optimizing a design, it is best first to optimize for the chosen voltage range of most

interest, using the Automatic setting. In order to subsequently evaluate performance of the circuit over a wider input voltage

range, fix the GI ratio in the Calculator input field, and then set the desired input voltage range.

Some components depend upon the switching frequency and the duty cycle. The switching frequency is regulated by the

ZXLD1371 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

≈

for Buck

for Boost

≈

for Buck-Boost

Equation 6

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.

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

for Buck

for Boost

=

for Buck-boost

Equation 7

= DC winding resistance of L1