ZXLD1371QESTTC Diodes Inc. / Zetex, ZXLD1371QESTTC Datasheet - Page 23

• Manufacturer Part Number: ZXLD1371QESTTC
• Description: MOSFET LED Controller TSSOP TSSOP-16EP T&R 2.5K
• Manufacturer: Diodes Inc. / Zetex

Specifications of ZXLD1371QESTTC

• Rohs: yes

ZXLD1371
Document number: DS35436 Rev. 1 - 2
Applications Information
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.
inductor/frequency setting and the MOSFET selection.
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
Because D must always be a positive number less than 1, these equations show that
This allows us to select the topology for the required voltage range.
More exact equations are used in the web Calculator. These are:
where V
V
V
V
V
R
D
D
D
D
D
D
OUT
OUT
OUT
F
DSON
COIL
BUCK
BOOST
BB
BUCK
BOOST
BB
< V
> V
> or = or < V
=
=
=
= schottky diode forward voltage, estimated for the expected coil current, I
= MOSFET drain source voltage in the ON condition (dependent on R
= DC winding resistance of L1
≈
≈
≈
IN
IN
V
V
V
V
OUT
V
V
OUT
OUT
V
OUT
OUT
OUT
IN
V
V
+ V
V
V
OUT
OUT
- V
+ V
OUT
+ V
V
IN
- V
IN
F
OUT
IN
+ V
F
+ I
IN
IN
+ I
+ I
+ V
OUT
F
+ V
IN
- V
IN
IN
F
+I
R
+ V
DSON
R
- V
S
OUT
(cont.)
S
+R
F
+R
DSON
- V
COIL
COIL
R
DSON
S
+R
+ V
COIL
F
Component selection begins with the current setting procedure, the
www.diodes.com
23 of 42
Finally after selecting the freewheeling diode and the output
for Buck (voltage step-down)
for Boost (voltage step-up)
for Buck-boost (voltage step-down or step-up)
for Buck
for Boost
for Buck-boost
for Buck
for Boost
for Buck-Boost
Diodes Incorporated
A Product Line of
DSON
COIL
and drain current = I
© Diodes Incorporated
Equation 7
February 2012
Equation 6
ZXLD1371
COIL
)