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參數(shù)資料
| 型號： | LM2791LD-L |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 顯示驅(qū)動器 |
| 英文描述： | Current Regulated Switched Capacitor LED Driver with Analog Brightness Control |
| 中文描述： | LED DISPLAY DRIVER, DSO10 |
| 封裝： | 3 X 3 MM, 0.80 MM HEIGHT, LLP-10 |
| 文件頁數(shù)： | 9/11頁 |
| 文件大?。?/td> | 226K |
| 代理商： | LM2791LD-L |
Application Information
(Continued)
turning-on its own parasitic diode and potentially latching-up.
As a result, it is important to select a schottky diode that will
carry at least 200mA or higher current to charge the output
capacitor during start-up. A schottky diode like 1N5817 can
be used for most applications or a surface mount diode such
as BAT54-series and MA2J704 used to reduce the circuit
size.
Table 2
below lists suggested schottky diode manufac-
tures.
TABLE 2. Diode Manufactures
Manufacturer
ON
Semiconductor
Phillips
Semiconductors
Panasonic
Semiconductor
Contact
(800) 344
3860
(800) 234
7381
(408) 945
5622
Schottky Diodes
www.onsemi.con
www.philipssemiconduc
tor.com
www.panasonic.com
LED SELECTION
The LM2791 is designed to drive LEDs with a forward volt-
age of about 3.0V to 4.0V or higher. The typical and maxi-
mum V
F
depends highly on the manufacturer and their tech-
nology.
Table 3
lists two suggested manufactures and
example part numbers. Each supplier makes many LEDs
that work well with the LM2791. The LEDs suggested below
are in a surface mount package and TOPLED or SIDEVIEW
configuration with a maximum forward current of 20mA.
These diodes also come in SIDELED or SIDEVIEW configu-
ration and various chromaticity groups. For applications that
demand color and brigthness matching, care must be taken
to select LEDs from the same chromaticity group. Forward
current matching is assured over the LED process variations
due to the constant current output of the LM2791. For best fit
selection for an application, consult the manufacturer for
detailed information.
TABLE 3. White LED Selection:
Component
LWT673/LWT67C
NSCW100/ NSCW215 Nichia
Manufacture Contact
Osram
www.osram-os.com
www.nichia.com
I
SET
PIN
An external resistor, R
SET
, sets the mirror current that is
required to provide a constant current through the LEDs. The
current through R
SET
and the LED is set by the internal
current mirror circuitry with a ratio of 25:1 The currents
through each LED are matched within 0.3%. R
should be
chosen not to exceed the maximum current delivery capa-
bility of the device. Table 4 shows a list of R
values when
maximun BRGT = 0V is applied. For other BRGT voltages,
R
SET
can be calculated using this formula:
R
SET
= (((BRGT
*
0.42) + V
OFFSET
))/(I
LED
)
*
25
TABLE 4. R
SET
Selections ( when BRGT pin = 0V)
I
LED
per LED
15mA
10mA
5mA
*R
SET
330
500
1K
* Use 1% resitor for Rset
Calculation of LED Current When Grounding BRGT:
V
IN
= 3.6V
V
OFFSET
= 200mV(Reference Voltage)
R
SET
= 330
I
LED
= ( V
OFFSET
/R
SET
)
*
25
I
LED
= (200mV/330)
*
25 = 15mA
BRGT PIN
The BRGT pin can be used to smoothly vary the brightness
of the White LEDs. In the LM2791, current on BRGT is
connected to an internal resistor divider which gives a factor
0.42 and summed with an offset voltage from the internal
bandgap (200mV). This voltage is fed to the operational
amplifier that controls the current through the mirror resistor
R
. The nominal range on BRGT is 0V to 3V. Care must
be taken to prevent voltages on BRGT that cause LED
current to exceed 36mA. Although this will not cause dam-
age to the IC, it will not meet the guaranteed specifications
listed in the Electrical Characteristics.
Table 5
shows the current through each LED for the LM2791
with various BRGT and R
SET
values.
Calculation of LED Current When BRGT Pin
>
0:
R
SET
= 2000
BRGT = 2.5V
V
OFFSET
= 200mV(Reference Voltage)
I
LED
= (((BRGT
*
0.42) + V
OFFSET
)/ R
SET
)
*
25
I
LED
= (((2.5
*
0.42) + 0.20)/2000 )
*
25 =15.6mA
TABLE 5. LED Current When Using BRGT Input (Values Highlighted in Boldface exceeded maximum current range of
the device if both LEDs are in use)
R
SET
(
)
BRGT (V)
0.5
1.0
1.5
2.0
2.5
3.0
1000
I
LED
(mA)
10.25
15.5
20.75
26
31.25
36.5
1500
I
LED
(mA)
6.84
10.3
13.8
17.3
20.80
24.3
2000
I
LED
(mA)
5.10
7.75
10.37
13.00
15.6
18.3
2500
I
LED
(mA)
4.1
6.2
8.3
10.4
12.5
14.6
L
www.national.com
9
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