ADT7110
Application Hints
▶
LED Current control
The LED current is determined by current ballast resistor
(R
LED1
~R
LED6
) and feedback resistor (R
FB2
). The current
setting procedure is described as below.
i) Choose the R
LED1
~ R
LED6
The voltage on current ballast resistor (R
LED
) is about
200mV normally. So the LED current is 200mV/R
LED
.
In order to set LED current accurately, the precision resistors
are preferred (1% recommended).
The R
LED
value as LED current is shown as below table.
< table 1 > R
LED
vs. LED current
R
LED
(Ω)
5.76
4.42
4.02
2.67
2.49
LED current (mA)
35
45
50
75
80
iii) Choose the R
FB1
(= R
FB1a
~ R
FB1f
)
R
FB1
is used to biasing of LED. And these resistor value
should be changed by the number of the LED branch.
Therefore you should choose appropriate R
FB1
value according
to the LED branch count. The R
FB1
selection formula is shown
as below.
R
FB1
≈
20
k
Ω ×
number of LED branches
Typical application circuit shows 6 LED branches and each
LED branch has 6 series LEDs. In this circuit, proper R
FB1
value is 120㏀ and R
FB1a
~ R
FB1f
value are used 120㏀ equally.
The R
FB1
value by the number of LED branch is shown below.
< table 3 > R
FB1
by the LED branches
Number of LED branch
6
5
4
3
2
1
R
FB1
(kΩ)
120
100
80
60
40
20
ii) Choose the R
FB2
Secondly you choose appropriate R
FB2
value for setting
V
LED
. R
FB2
is determined by forward voltage of the 6 series
LEDs because the forward voltages of LED are different each
other according to the LED manufacturers. (1.2 ~ 1.5volts for
V
F
setting current flow away). The table and formula of the
R
FB2
are shown as below.
< table 2 > R
FB2
vs. VLED
R
FB2
(kΩ)
61.9
64.9
66.5
68.1
69.8
71.5
73.2
75.0
VLED (V)
8.4
8.6
8.8
9.0
9.2
9.4
9.6
9.8
▶
LED current check
The accurate method of measuring LED current is to measure
the voltage on current ballast resistor (R
LED
). And then the
LED current is simply obtained by dividing this voltage by
R
LED
.
To measure voltage on R
LED
accurately, (-) probe of the
voltage meter is connected to (-) terminal of the ballast resistor
and measure the voltage of (+) terminal on ballast resistor.
Fig. 2 shows the method of measuring voltage on ballast
resistor.
LED current= V(R
LED1
)/R
LED1
V(R
LED1
)
R
LED1
(+)
(-)
Voltage
meter
R
FB
2
VLED
−
V
FB
≈
100uA
<Fig. 2 LED current test>
It is possible to calculate the LED current by measuring of ICC
current. As the ADT7110 is basically buck converter, its ICC
current is the function of VIN, VOUT, LED branch current and
quiescent current. Therefore it is not easy to calculate accurate
LED current by measurement of ICC current.
The relation LED branch current to the ICC current is shown
below formula.
V
1
I
LED
.
1branch
≈
ICC
×
efficiency
×
IN
×
,
where
V
OUT
N
Where the VLED is the sum of the forward voltage in 6 series
LEDs at setting current and V
FB
is the feedback voltage.
(typically 2.21V)
Fig. 1 shows the method of measuring VLED.
VLED
Setting current
N
:
the number of LED branch
efficiency
:
efficiency of the ADT7110 at applied condition
<Fig. 1 VLED test>
* This specifications are subject to be changed without notice
Jun. 07. 2008 / Rev. 0.1
6/8
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