Diodes ZXLD1371 - Manual (Page 23)

ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
23 of 42
www.diodes.com
February 2012
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Applications Information (cont.)
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
D
BUCK
≈
V
OUT
V
IN
for Buck
D
BOOST
≈
V
OUT
- V
IN
V
OUT
for Boost
D
BB
≈
V
OUT
V
OUT
+ V
IN
for Buck-Boost
Equation 6
Because D must always be a positive number less than 1, these equations show that
V
OUT
< V
IN
for Buck (voltage step-down)
V
OUT
> V
IN
for Boost (voltage step-up)
V
OUT
> or = or < V
IN
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:
D
BUCK
=
V
OUT
+ V
F
+ I
OUT
R
S
+R
COIL
V
IN
+ V
F
- V
DSON
for Buck
D
BOOST
=
V
OUT
- V
IN
+ I
IN
R
S
+R
COIL
+ V
F
V
OUT
+ V
F
- V
DSON
for Boost
D
BB
=
V
OUT
+ V
F
+ I
IN
+I
OUT
R
S
+R
COIL
V
OUT
+ V
IN
+ V
F
- V
DSON
for Buck-boost
Equation 7
where V
F
= schottky diode forward voltage, estimated for the expected coil current, I
COIL
V
DSON
= MOSFET drain source voltage in the ON condition (dependent on R
DSON
and drain current = I
COIL
)
R
COIL
= DC winding resistance of L1