Diodes ZXLD1374 - Manual (Page 17)

ZXLD1374
Document number: DS35032 Rev. 3 - 2
17 of 39
www.diodes.com
September 2012
© Diodes Incorporated
ZXLD1374
A Product Line of
Diodes Incorporated
Application Information
(cont.)
⎟
⎠
⎞
⎜
⎝
⎛
+
=
2 RGI 1 RGI
1 RGI
ADJ _ GI Equation 2 (Boost and Buck-Boost modes)
The control loop sets the duty cycle so that the sense resistor current is
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟
⎠
⎞
⎜
⎝
⎛
−
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
V
V
D 1
ADJ _ GI
R
225 . 0
I
REF
ADJ
S
RS
Equation 3 (Boost and Buck-Boost modes)
I
RS
equals the coil current. The coil is connected only to the switch and the schottky diode. The schottky diode passes the LED current.
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
Equation 4 (Boost and Buck-Boost)
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 R I V
REF
ADJ
S RS RS
Equation 5 (Boost and Buck-boost)
This shows that the sense voltage varies with duty cycle in Boost and Buck-boost configurations.
Application Circuit Designs
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 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 ZXLD1374 to a large
extent, depending upon conditions. This is discussed in a later paragraph dealing with coil selection.