ZXLD1371Q

ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

22 of 42

www.diodes.com

September 2015

© Diodes Incorporated

ZXLD1371Q

0.225

S

GI ADJ

(Boost and Buck-boost)

Equation 4

This shows that the LED current depends on the ADJ pin voltage, the reference voltage and 3 resistor values (RS, RGI1 and RGI2). It is

independent of the input and output voltages.

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

0.225

S

GI ADJ

(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.

GI ADJ

1-D

(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 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 ZXLD1371Q 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

S

COIL

V

for Boost

S

COIL

for Buck-boost

Equation 7

where

= DC winding resistance of L1