LM2794 datasheet(10/21 Pages) TI1

Part #	LM2794

Description	Current Regulated Switched Capacitor LED Supply with Analog and PWM Brightness Control

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Manufacturer	TI1 [Texas Instruments]

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Amps

1

10

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188

BRGT

V

*

+

385

.

0

.

0

SET

R

*

o

MirrorRati

OFFSET

V

+

BRGT

V

385

.

0

*

R

SET

LED =

I

LED =

I

LM2794, LM2795

SNVS168K – MAY 2004 – REVISED MAY 2004

www.ti.com

In addition to RSET, a voltage may be applied to the VBRGT pin to vary the LED current. By adjusting current with

the Brightness pin (BRGT), the brightness of the LEDs can be smoothly varied.

Applying a voltage on BRGT between 0 to 3 volts will linearly vary the LED current. Voltages above 3V do not

increase the LED current any further. The voltage on the VBRGT pin is fed into an internal resistor network with a

ratio of 0.385. The resulting voltage is then summed with a measured offset voltage of 0.188V, which comes

from the reference voltage being fed through a resistor network (See Functional Block Diagram). The

brightness control circuitry then uses the summed voltage to control the voltage across RSET. An equation for

approximating the LED current is:

ILED CURRENT SELECTION PROCEDURES

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 ILED current. Use the ILED equation to calculate RSET by setting BRGT to 0V

or use Table 3 to select a value for RSET when BRGT 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 brightness control over the full duty cycle adjustment range,

the PWM frequency (f) should be limited to accommodate the turn-on time (TON = 100µs) of the device.D ×

(1/f) > TONfMAX = DMIN ÷ TONIf 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 ILED

being half of the programmed LED current. For example, if RSET is set to program 15mA, a 50% duty cycle

will result in an average ILED of 7.5mA. For the LM2795 however, 100% duty cycle will turn off the LEDs and

a 50% duty cycle will result in an average ILED being half the programmed LED current.

Brightness Control Using BRGT

1. Choose the maximum ILED desired and determine the 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 RSET required or use the ILED equation above to calculate RSET.

3. Use Table 4 as a reference for the dimming profile of the LEDs, when BRGT ranges from 0V to 3V.

Table 3. RSET Values

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

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

RSET Values

BRGT

2.67K

Ω

1.33K

Ω

909

Ω

665

Ω

0.0V

0.7mA

1.4mA

2.1mA

2.8mA

10

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