Zxld1374, Application information – Diodes ZXLD1374 User Manual
Page 21
ZXLD1374
Document number: DS35032 Rev. 3 - 2
21 of 39
September 2012
© Diodes Incorporated
ZXLD1374
A Product Line of
Diodes Incorporated
Application 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
:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ −
=
ADJ
_
GI
ADJ
_
GI
1
R
R
1
GI
1
GI
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
(
) (
)
6875
.
0
4
.
38
12
4
.
38
V
V
V
OUT
IN
OUT
=
−
=
−
From Equation 16, we set GI_ADJ to 1 – D = 0.3125.
If R
GI1
= 33kΩ, then from Equation 17, R
GI2
= 33000 * ( 1 -0.3125 ) / 0.3125 = 72.6kΩ. Let us choose the preferred value R
GI2
= 75kΩ. Now
GI_ADJ is adjusted to the new value, using Equation 2.
305
.
0
K
75
k
33
k
33
2
RGI
1
RGI
1
RGI
ADJ
_
GI
=
+
=
⎟
⎠
⎞
⎜
⎝
⎛
+
=
Now we calculate Rs from Equation 11. Assume ADJ is connected to REF.
Ω
=
=
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
196
.
0
305
.
0
*
35
.
0
225
.
0
V
V
ADJ
_
GI
I
225
.
0
R
REF
ADJ
LED
SBoostBB
A preferredvalue of R
SBoostBB
= 0.2Ω 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.