LM3533 Datasheet, PDF(18/53 Page) Texas Instruments – Complete Lighting Power Solution for Smartphone Handsets

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LM3533 Datasheet, PDF (18/53 Pages) Texas Instruments – Complete Lighting Power Solution for Smartphone Handsets

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LM3533

LOW-VOLTAGE LED BIASING

Each low-voltage LED can be powered from the LM3533âs charge pump output (CPOUT) or from an external source. When powered

from CPOUT the anode connect bit (Anode Connect Register bits [6:2]) for that particular low-voltage current sink must be set to

'1' (default). This allows for the specific low-voltage current sink to have control over the charge pumps gain control (see AUTO-

MATIC GAIN section).

When powered from alternate sources (such as VIN) the anode connect bit for the particular low-voltage current sink must be set

to '0'. This removes the particular current sink from the charge pump feedback loop. In these configurations the application must

ensure that the headroom voltage across the low-voltage current sink is high enough to prevent the low-voltage current sinks from

going into dropout (see Typical Performance Characteristics for data on the low-voltage LED current vs headroom voltage).

The LVLEDX Anode Connect bits also determine how the Shorted low-voltage LED String fault flag is triggered (see Fault Flags/

Protection Features).

LED Current Mapping Modes

All control banks can be programmed for either exponential or linear mapping modes (see Figure 5 ). These modes determine the

transfer characteristic of backlight code to LED current.

EXPONENTIAL MAPPING

In Exponential Mapping Mode the brightness code to backlight current transfer function is given by the equation:

Where ILED_FULLSCALE is the full-scale LED current setting (see Table 11), Code is the backlight code in the brightness register, and

DPWM is the PWM input duty cycle. In Exponential Mapping Mode the current ramp (either up or down) appears to the human eye

as a more uniform transition then the linear ramp. This is due to the logarithmic response of the eye.

LINEAR MAPPING

In Linear Mapping Mode the brightness code to backlight current has a linear relationship and follows the equation:

Where ILED_FULLSCALE is the full-scale LED current setting, Code is the backlight code in the brightness register, and DPWM is the

PWM input duty cycle.

30135791

FIGURE 5. LED Current Mapping Modes

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