LM2755

SNVS433D – OCTOBER 2007 – REVISED APRIL 2013

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Table 1. Internal Registers of LM2755 (continued)

Register Name

Internal Hex Address

Power On Value

D2 Time High: nhigh2

xB3

0000 0000

D2 Ramp-Down Step Time: nfall2

xB4

0000 0000 (1)

D2 Timing: nlow2

xB2

0000 0000

D3 High Level: nID3H

xC9

1110 0000

D3 Low Level: nID3L

xC8

1110 0000

D3 Delay: ndelay3

xC1

0000 0000

D3 Ramp-Up Step Time: nrise3

xC5

0000 0000 (1)

D3 Time High: nhigh3

xC3

0000 0000

D3 Ramp-Down Step Time: nfall3

xC4

0000 0000 (1)

D3 Timing: nlow3

xC2

0000 0000

GENERAL PURPOSE REGISTER DESCRIPTION

•

Bit 0: enable output D1 with high current level.

•

Bit 1: enable output D2 with high current level.

•

Bit 2: enable output D3 with high current level.

•

Bit 3: enable dimming waveform on output D1.

•

Bit 4: enable dimming waveform on output D2.

•

Bit 5: enable dimming waveform on output D3.

•

Bit 6: enable external clock. '1' = External Clock Sync, '0' = Internal Clock Used

•

Bit 7: If Bit 7 = 0 the charge pump is powered on before any dimming waveform is enabled. It is

recommended that Bit7 be set to a '0' if an external clock is used. If Bit 7 = 1 the dimming waveform can be

enabled before charge pump is powered on.

Application Information

SETTING FULL-SCALE LED CURRENT

The current through the LEDs connected to D1, D2 and D3 can be set to a desired level simply by connecting an

appropriately sized resistor (RSET) between the ISET pin of the LM2755 and GND. The LED currents are

proportional to the current that flows through the ISET pin and are a factor of 200 times greater than the ISET

currents. The feedback loop of the internal amplifier sets the voltage of the ISET pin to 1.25V (typ.). The

statement above is simplified in the equation below:

IDx (Full-Scale) = 200 × (VISET / RSET)

(1)

Please refer to I2C-Compatible Interface for detailed instructions on how to adjust the brightness control

registers.

BRIGHTNESS LEVEL CONTROL

Once the desired RSET value has been chosen, the LM2755 can internally dim the LEDs by modulating the

currents with an internally set 20kHz PWM signal. The PWM duty cycle percentage is independently set for each

LED through the I2C-compatible interface. The 32 brightness levels follow a exponentially increasing pattern

rather than a linearly increasing one in order to better match the human eye's response to changing brightness.

The brightness level response is modeled in the following equations.:

IDx LOW = (0.9)

(31-n

IDxL

) × I

Dx Fullscale

(2)

IDx HIGH = (0.9)

(31-n

IDxH

) × I

Dx Fullscale

(3)

nIDxH and nIDxL are numbers between 0 and 31 stored in the Brightness Level registers. When the waveform

enable bits are set to '1', nIDxH and nIDxL are the brightness level boundaries. These equations apply to all Dx

outputs and their corresponding registers. A '0' code in the Brightness Control register sets the current to an "off-

state" (0mA).

12

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