I

The following procedures illustrate how to set and adjust

output current levels. For constant brightness or analog

brightness control, go to “Brightness control using BRGT”.

Otherwise refer to “Brightness control using PWM”.

Brightness Control Using PWM

1.

Set the BRGT pin to 0V.

2.

Determine the maximum desired I

I

use Table 3 to select a value for R

equals 0V.

3.

Brightness control can be implemented by pulsing a

signal at the SD pin. LED brightness is proportional to

the duty cycle (D) of the PWM signal. For linear bright-

ness control over the full duty cycle adjustment range,

the PWM frequency (f) should be limited to accommo-

date the turn-on time (T

ON

f

If the PWM frequency is much less than 100Hz, flicker

may be seen in the LEDs. For the LM2794, zero duty

cycle will turn off the LEDs and a 50% duty cycle will

result in an average I

LED current. For example, if R

15mA, a 50% duty cycle will result in an average I

7.5mA. For the LM2795 however, 100% duty cycle will

turn off the LEDs and a 50% duty cycle will result in an

average I

Brightness Control Using BRGT

1.

Choose the maximum I

max voltage to be applied to the BRGT pin. For constant

brightness, set BRGT to a fixed voltage between 0V to

3V.

2.

Use Table 3 to determine the value of R

use the I

3.

Use Table 4 as a reference for the dimming profile of the

LEDs, when BRGT ranges from 0V to 3V.

TABLE 3. R

LED Current

BRGT

5mA

10mA

15mA

20mA

0.0V

374

Ω

187

Ω

124

Ω

93.1

Ω

0.5V

768

Ω

383

Ω

255

Ω

191

Ω

1.0V

1.15K

Ω

576

Ω

383

Ω

287

Ω

1.5V

1.54K

Ω

768

Ω

511

Ω

383

Ω

2.0V

1.91K

Ω

953

Ω

624

Ω

475

Ω

LED Current

BRGT

5mA

10mA

15mA

20mA

2.5V

2.32K

Ω

1.15K

Ω

768

Ω

576

Ω

3.0V

2.67K

Ω

1.33K

Ω

909

Ω

665

Ω

TABLE 4. LED Current

R

BRGT

2.67K

Ω

1.33K

Ω

909

Ω

665

Ω

0.0V

0.7mA

1.4mA

2.1mA

2.8mA

0.5V

1.4mA

2.9mA

4.2mA

5.7mA

1.0V

2.1mA

4.3mA

6.3mA

8.6mA

1.5V

2.9mA

5.8mA

8.4mA

11.5mA

2.0V

3.6mA

7.2mA

10.5mA

14.4mA

2.5V

4.3mA

8.7mA

12.7mA

17.3mA

3.0V

5.0mA

10.1mA

14.8mA

20.2mA

CHARGE PUMP OUTPUT (P

The LM2794/5 charge pump is an unregulated switched

capacitor converter with a gain of 1.5. The voltage at the

output of the pump (the P

rail can be used to deliver additional current to other circuitry.

Figure 2 shows how to connect additional LEDs to P

ballast resistor sets the current through each LED, and LED

current matching is dependent on the LED forward voltage

matching. Because of this, LEDs driven by P

mended for functions where brightness matching is not criti-

cal, such as keypad backlighting.

Since P

usually practical only with a fixed input voltage. If the input

voltage is not fixed (Li-Ion battery, for example), using a

linear regulator between the P

recommended. National Semiconductor’s LP3985-4.5V low-

dropout linear regulator is a good choice for such an appli-

cation.

The voltage at P

supplied to the LM2794/5, the total LM2794/5 output current,

and the output resistance (R

pump. Output resistance is a model of the switching losses

of the charge pump. Resistances of the internal charge

pump switches (MOS transistors) are a primary component

of the LM2794/5 output resistance. Typical LM2794/5 output

resistance is 3.0

Ω. For worst-case design calculations, using

an output resistance of 3.5

Ω is recommended. (Worst-case

recommendation accounts for parameter shifts from part-to-

part variation and applies over the full operating temperature

range).

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