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

Contact

Schottky Diodes

ON

Semiconductor

(800) 344

3860

www.onsemi.con

Phillips

Semiconductors

(800) 234

7381

www.philipssemiconduc

tor.com

Panasonic

Semiconductor

(408) 945

5622

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

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

Manufacture Contact

LWT673/LWT67C

Osram

www.osram-os.com

NSCW100/ NSCW215 Nichia

www.nichia.com

I

An external resistor, R

required to provide a constant current through the LEDs. The

current through R

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

R

TABLE 4. R

I

*R

SET

15mA

330

Ω

10mA

500

Ω

5mA

1K

* Use 1% resitor for Rset

Calculation of LED Current When Grounding BRGT:

V

V

R

Ω

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

R

Ω

BRGT = 2.5V

V

I

I

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

Ω)

1000

Ω

1500

Ω

2000

Ω

2500

Ω

BRGT (V)

I

I

I

I

0.5

10.25

6.84

5.10

4.1

1.0

15.5

10.3

7.75

6.2

1.5

20.75

13.8

10.37

8.3

2.0

26

17.3

13.00

10.4

2.5

31.25

20.80

15.6

12.5

3.0

36.5

24.3

18.3

14.6

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