Diodes ZXLD1371 - Manual (Page 26)

ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
26 of 42
www.diodes.com
February 2012
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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
) Equation 15
An optimum compromise for GI_ADJ has been suggested, i.e.
GI_ADJ
AUTO
= 1 - D
MAX
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 Ω, then from Equation 17, R
GI2
= 33000 * ( 1 -0.3125 ) / 0.3125 = 72.6k Ω. Let us choose the preferred value
R
GI2
= 75k Ω. 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 Ω
A preferred value of R
SBoostBB
= 0.2 Ω 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 RGI1 RG2
0.2 30k
Ω 120kΩ
0.25 33k
Ω 100kΩ
0.3 39k
Ω 91kΩ
0.35 30k
Ω 56kΩ
0.4 100k
Ω 150kΩ
0.45 51k
Ω 62kΩ
0.5 30k
Ω 30kΩ
This completes the LED current setting.