Diodes ZXLD1370 - Manual (Page 16)

ZXLD1370
Document number: DS32165 Rev. 5 - 2
16 of 39
www.diodes.com
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
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
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
ADJ _ GI
R
225 . 0
I
S
LED ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
= (Boost and Buck-Boost)
Now I
LED
is dependent only on the 3 resistor values.
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 web site. 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
BUCK
≈ 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
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.