LM2791LD-H/NOPB National Semiconductor, LM2791LD-H/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM2791LD-H/NOPB
• Description: IC LED DRIVR WHITE BCKLGT 10-LLP
• Manufacturer: National Semiconductor
• Type: Backlight, White LEDr

Specifications of LM2791LD-H/NOPB

• Constant Current: Yes
• Topology: PWM, Switched Capacitor (Charge Pump)
• Number Of Outputs: 2
• Internal Driver: Yes
• Type - Primary: Backlight
• Type - Secondary: White LED
• Frequency: 450kHz ~ 850kHz
• Voltage - Supply: 3 V ~ 5.8 V
• Mounting Type: Surface Mount
• Package / Case: 10-LLP
• Operating Temperature: -30°C ~ 85°C
• Current - Output / Channel: 36mA
• Internal Switch(s): Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Efficiency: -
• Other names: LM2791LD-H; LM2791LD-HTR

Application Information
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.
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
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.
I
An external resistor, R
required to provide a constant current through the LEDs. The
current through R
TABLE 5. LED Current When Using BRGT Input (Values Highlighted in Boldface exceeded maximum current range of
SET
Component
LWT673/LWT67C
NSCW100/ NSCW215 Nichia
Semiconductors
Semiconductor
Semiconductor
Manufacturer
Panasonic
PIN
Phillips
F
ON
depends highly on the manufacturer and their tech-
TABLE 3. White LED Selection:
TABLE 2. Diode Manufactures
BRGT (V)
SET
R
SET
(800) 344
(800) 234
(408) 945
Contact
0.5
1.0
1.5
2.0
2.5
3.0
3860
7381
5622
and the LED is set by the internal
SET
(Ω)
Manufacture Contact
Osram
, sets the mirror current that is
www.philipssemiconduc
www.panasonic.com
I
LED
Schottky Diodes
www.osram-os.com
1000Ω
www.onsemi.con
10.25
20.75
31.25
15.5
36.5
www.nichia.com
26
(Continued)
(mA)
tor.com
the device if both LEDs are in use)
I
LED
1500Ω
20.80
6.84
10.3
13.8
17.3
24.3
(mA)
9
current mirror circuitry with a ratio of 25:1 The currents
through each LED are matched within 0.3%. R
chosen not to exceed the maximum current delivery capa-
bility of the device. Table 4 shows a list of R
maximun BRGT = 0V is applied. For other BRGT voltages,
R
Calculation of LED Current When Grounding BRGT:
V
V
R
I
I
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
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
Calculation of LED Current When BRGT Pin
R
BRGT = 2.5V
V
I
I
LED
LED
LED
LED
IN
OFFSET
OFFSET
SET
SET
SET
SET
* Use 1% resitor for Rset
TABLE 4. R
= 3.6V
= ( V
= (200mV/330)
= (((BRGT
= (((2.5
. The nominal range on BRGT is 0V to 3V. Care must
can be calculated using this formula:
= 330Ω
= 2000Ω
R
SET
= 200mV(Reference Voltage)
= 200mV(Reference Voltage)
I
LED
OFFSET
I
LED
= (((BRGT
*
2000Ω
15mA
10mA
10.37
13.00
0.42) + 0.20)/2000 )
5mA
5.10
7.75
15.6
18.3
per LED
(mA)
SET
*
/R
0.42) + V
SET
Selections ( when BRGT pin = 0V)
*
25 = 15mA
)
*
*
0.42) + V
25
SET
OFFSET
values.
I
LED
2500Ω
*
)/ R
10.4
12.5
14.6
OFFSET
25 =15.6mA
4.1
6.2
8.3
(mA)
SET
*R
330Ω
500Ω
))/(I
1K
)
SET
SET
*
LED
25
SET
values when
>
www.national.com
)
*
should be
0:
25