ADT7110 ADTech, ADT7110 Datasheet - Page 6

• Manufacturer Part Number: ADT7110
• Description: fixed frequency step-down converter
• Manufacturer: ADTech

www.DataSheet.in
Jun. 07. 2008 / Rev. 0.1
Application Hints
▶ LED Current control
(R
setting procedure is described as below.
i) Choose the R
200mV normally. So the LED current is 200mV/R
In order to set LED current accurately, the precision resistors
are preferred (1% recommended).
The R
< table 1 > R
ii) Choose the R
V
LEDs because the forward voltages of LED are different each
other according to the LED manufacturers. (1.2 ~ 1.5volts for
V
R
< table 2 > R
Where the VLED is the sum of the forward voltage in 6 series
LEDs at setting current and V
(typically 2.21V)
Fig. 1 shows the method of measuring VLED.
The LED current is determined by current ballast resistor
LED
F
FB2
The voltage on current ballast resistor (R
Secondly you choose appropriate R
LED1
R
setting current flow away). The table and formula of the
Setting current
FB
. R
are shown as below.
LED
~R
2
R
FB2
LED
LED6
≈
5.76
4.42
4.02
2.67
2.49
value as LED current is shown as below table.
is determined by forward voltage of the 6 series
VLED
(Ω)
) and feedback resistor (R
LED
FB2
R
LED1
100uA
FB2
FB2
vs. VLED
vs. LED current
61.9
64.9
66.5
68.1
69.8
71.5
73.2
75.0
(kΩ)
~ R
−
<Fig. 1 VLED test>
VLED
V
LED6
FB
LED current (mA)
* This specifications are subject to be changed without notice
35
45
50
75
80
FB
VLED (V)
is the feedback voltage.
8.4
8.6
8.8
9.0
9.2
9.4
9.6
9.8
FB2
FB2
value for setting
). The current
LED
LED
) is about
.
6/8
iii) Choose the R
should be changed by the number of the LED branch.
Therefore you should choose appropriate R
to the LED branch count. The R
as below.
Typical application circuit shows 6 LED branches and each
LED branch has 6 series LEDs. In this circuit, proper R
value is 120㏀ and R
The R
< table 3 > R
▶ LED current check
The accurate method of measuring LED current is to measure
the voltage on current ballast resistor (R
LED current is simply obtained by dividing this voltage by
R
To measure voltage on R
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.
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.
efficiency
I
N
LED
R
LED
FB1
:
.
the
.
1
FB1
branch
is used to biasing of LED. And these resistor value
number
value by the number of LED branch is shown below.
R
≈
:
FB1
efficiency
Number of LED branch
FB1
ICC
V(R
<Fig. 2 LED current test>
≈ 20
FB1
by the LED branches
of
LED current= V(R
×
LED1
(= R
efficiency
FB1a
LED
k
)
Ω
6
5
4
3
2
1
of
FB1a
~ R
×
http://www.ad-tech.co.kr
branch
the
number
LED
FB1f
~ R
×
ADT7110
V
FB1
FB1f
V
accurately, (-) probe of the
OUT
value are used 120㏀ equally.
IN
selection formula is shown
)
of
LED1
×
N
(+)
1
(-)
R
LED
)/R
FB1
at
120
100
80
60
40
20
LED
,
Voltage
FB1
(kΩ)
LED1
meter
applied
ADT7110
where
). And then the
branches
value according
condition
FB1