be used for most applications or a surface mount diode such

as BAT54-series and MA2J704 can be used to reduce the

circuit size.Table 2 below lists suggested schottky diode

manufactures.

TABLE 2. Diode Manufactures

Manufacturer

Contact

website

ON

Semiconductor

(800) 344

3860

www.onsemi.com

Phillips

Semiconductors

(800) 234

7381

www.philipssemi-

conductor.com

Panasonic

Semiconductors

(408) 945

5622

www.panasonic.com

LED SELECTION

The LM2792 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 LM2792. 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 LM2792. 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 3 shows a list of R

maximun BRGT=3V is applied. For other BRGT voltages,

R

R

TABLE 4. R

maximum)

I

R

SET

15mA

2.1K

Ω

10mA

3.15K

Ω

5mA

6.3K

Ω

BRGT PIN

The BRGT pin can be used to smoothly vary the brightness

of the LEDs. In the LM2792, current on BRGT is connected

to an internal resistor divider which gives a factor of 0.42

(see Figure 1) . This voltage is fed to the operational ampli-

fier that controls the current through the mirror resistor R

The nominal range on BRGT is 0V to 3V.

This means some current must be provided on the BRGT pin

or no current will flow through the LEDs. The LM2792 can

provide an infinite ratio, from fully off (essentially zero cur-

rent) to the maximum current set by the R

must be taken to prevent voltages on BRGT that cause LED

current to exceed a total of 34mA. Although this will not

cause damage to the IC, it will not meet the guaranteed

specifications listed in the Electrical Characteristics.

Calculation of LED Current When Using BRGT :

V

R

Ω

I

I

Note that making V

∼ = 0mA

BRIGHTNESS CONTROL USING PWM

Brigthness control can be implemented by pulsing a signal at

the SD pin. The recommended signal should be between

100Hz to 1kHz. If the operating PWM frequency is much less

than 100Hz, flicker may be seen in the LEDs. Likewise, if

frequency is much higher, brightness in the LEDs will not be

linear. When a PWM signal is used to drive the SD pin of the

LM2792, connect BRGT pin to a maximun of 3V to ensure

the widest range. Similarly, the voltage at the BRGT pin can

be set higher than 3V without damage to the IC, it will not

increase the brigthness of the LED significantly. R

is selected using the above I

used. The brigthness is controlled by increasing and de-

creasing the duty cycle of the PWM. Zero duty cycle will turn

off the brigthness and a 50% duty cycle waveform produces

an average current of 7.5mA if R

maximum LED current of 15mA. So the LED current varies

linearly with the duty cycle.

PARALLEL DX OUTPUTS FOR INCREASED CURRENT

DRIVE

Outputs D1 and D2 may be connected together to drive a

single LED. In such a configuration, two parallel current

sources of equal value drive the single LED. R

V

the outputs is programmed to 50% of the total desired LED

current. For example, if 30mA is the desired drive current for

the single LED, R

the current through each of the outputs is 15mA. Connecting

the outputs in parallel does not affect internal operation of

the LM2792 and has no impact on the Electrical Character-

istics and limits previously presented. The available Dx out-

put current, maximum Dx voltage, and all other specifica-

tions provided in the Electrical Characteristics table apply to

this parallel output configuration, just as they do to the

standard 2-LED application circuit.

THERMAL PROTECTION

The LM2792 has internal thermal protection circuitry to dis-

able the charge pump if the junction temperature exceeds

150˚C. This feature will protect the device from damage due

to excessive power dissipation. The device will recover and

operate normally when the junction temperature falls below

the maximum operating junction temperature of 100˚C. It is

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