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ZXLD1371
Applications Information (cont.)
This will give small enough I LED error for most practical purposes. Satisfactory operation will be obtained if V RS is more than
about 80mV. This means GI_ADJ should be greater than (1-D MIN ) * 80/225 = (1- D MIN ) * 0.355.
There is also a maximum limit on V RS which gives a maximum limit for GI_ADJ. If V RS exceeds approximately 300mV, or
133% of 225mV, the STATUS output may indicate an over-current condition. This will happen for larger D MAX . Therefore,
together with the requirement of Equation 13 , the recommended range for GI_ADJ is
0.355 ( 1-D MIN ) < GI_ADJ < 1.33 ( 1-D MAX )
An optimum compromise for GI_ADJ has been suggested, i.e.
GI_ADJ AUTO = 1 - D MAX
Equation 15
Equation 16
This value has been used for the “Automatic” setting of the web Calculator. If 1-D MAX is less than 0.2, then GI_ADJ is set to
0.2. If 1- D MAX is greater than 0.5 then GI_ADJ is set to 0.5.
Once GI_ADJ has been selected, a value of RGI1 can be selected from Equation 12 . Then RGI2 is calculated as follows,
rearranging Equation 2 :
R GI2 = R GI1
1-GI_ADJ
GI_ADJ
Equation 17
For example to drive 12 LEDS at a current of 350mA from a 12V supply requires Boost configuration. Each LED has a
forward voltage of 3.2V at 350mA, so Vout = 3.2*12 = 38.4V. From Equation 6 , the duty cycle is approximately
V OUT -V IN
V OUT
=
38.4-12
38.4
= 0.6875
From Equation 16 , we set GI_ADJ to 1 – D = 0.3125.
IF R GI1 = 33k ? , then from Equation 17 , R GI2 = 33000 * ( 1 -0.3125 ) / 0.3125 = 72.6k ? . Let us choose the preferred value
R GI2 = 75k ? . Now GI_ADJ is adjusted to the new value, using Equation 2 .
GI_ADJ =
RGI1
RGI1 +RGI2
=
33k
33k +75k
=0.305
Now we calculate Rs from Equation 11 . Assume ADJ is connected to REF.
R SBoostBB =
0.225
I LED
GI_ADJ
V ADJ
V REF
=
0.225
0.35
* 0.305 = 0.196 ?
A preferred value of R SBoostBB = 0.2 ? will give the desired LED current with an error of 2% due to the preferred value
selection.
Table 1 shows typical resistor values used to determine the GI_ADJ ratio with E24 series resistors.
Table 1
GI ratio
0.2
0.25
0.3
0.35
0.4
0.45
0.5
RGI1
30k Ω
33k Ω
39k Ω
30k Ω
100k Ω
51k Ω
30k Ω
RG2
120k Ω
100k Ω
91k Ω
56k Ω
150k Ω
62k Ω
30k Ω
This completes the LED current setting.
ZXLD1371
Document number: DS35436 Rev. 1 - 2
26 of 42
www.diodes.com
February 2012
? Diodes Incorporated
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