LM3533 Datasheet, PDF (19/53 Pages) Texas Instruments – Complete Lighting Power Solution for Smartphone Handsets

LM3533
LED Current Ramping
STARTUP/SHUTDOWN RAMP
The startup and shutdown ramp times are independently programmable in the Startup/Shutdown Transition Time Register (see
Table 4). There are 8 different Startup and 8 different Shutdown times. The startup times can be programmed independently from
the shutdown times, but teach Control bank is not independently programmable. For example, programming a startup or shutdown
time will not affect the already pre-programmed ramp time for each Control Bank.
The startup ramp time is from when the Control Bank is enabled to when the LED current reaches its initial set point. The shutdown
ramp time is from when the Control Bank is disabled to when the LED current reaches 0.
RUN-TIME RAMP
Current ramping from one brightness level to the next is programmed via the Run-Time Transition Time Register (see Table 5).
There are 8 different ramp-up times and 8 different ramp-down times. The ramp-up time can be programmed independently from
the ramp-down time, but each Control Bank cannot be independently programmed. For example, programming a ramp-up or ramp-
down time is a global setting for all Control Banks.
Brightness Register Current Control
For simple user-adjustable current control, the LM3533 features Brightness Register Current Control. This mode is selected via
the Control Bank Brightness Configuration Registers (see Table 8 and Table 10). Once set for Brightness Register Current Control,
the LED current is set by writing directly to the appropriate Control Bank Brightness Registers (see Table 28). In this mode the
current for a particular Control Bank becomes a function of the full-scale LED current, the 8-bit code in the respective brightness
register, and the PWM input duty cycle (if PWM is enabled). The Control Bank Brightness Register contains an 8-bit code which
represents the percentage of the full-scale LED current. This percentage of full-scale current is different depending on the selected
mapping mode (see LED Current Mapping Modes).
PWM Control
The LM3533’s PWM input can be enabled for any of the Control Banks (see Table 7). Once enabled, the LED current becomes a
function of the code in the Control Bank Brightness Configuration Register and the PWM input-duty cycle.
The PWM input accepts a logic level voltage and internally filters it to an analog control voltage. This results in a linear response
of duty cycle to current, where 100% duty cycle corresponds to the programmed brightness code multiplied by the Full-Scale Current
setting.
PWM INPUT FREQUENCY RANGE
The usable input frequency range for the PWM input is governed on the low end by the cutoff frequency of the internal low-pass
filter (540Hz, Q = 0.33) and on the high end by the propagation delays through the internal logic. For frequencies below 2kHz the
current ripple begins to become a larger portion of the DC LED current. Additionally, at lower PWM frequencies the boost output
voltage ripple increases, causing a non-linear response from the PWM duty cycle to the average LED current due to the response
time of the boost. For the best response of current vs. duty cycle, the PWM input frequency should be kept between 2kHz and
100kHz.
PWM INPUT POLARITY
The PWM Input can be set for active low polarity, where the LED current is a function of the negative duty cycle. This is set via the
OVP/Boost Frequency/PWM Polarity Register (see ).
ALS Current Control
The LM3533 features Ambient Light Sensor (ALS) current control which allows the LED current to be automatically set based upon
the received ambient light. To implement ambient light current control the LM3533 uses a 5 brightness zone implementation with
3 sets of Zone Targets.
ALS BRIGHTNESS ZONES (ZONE BOUNDARIES)
The LM3533 provides for a 5 brightness zone ambient light sensor interface. This allows for the LED current in any current sink to
change based upon which zone the received ambient light falls into. The brightness zones are configured via 4 ALS Zone Boundary
High and 4 ALS Zone Boundary Low Registers. Each Zone Boundary register is 8 bits with a full-scale voltage of 2V. This gives a
2V/255 = 7.843mV per bit. Figure 7 shows the mapping from the ALS Brightness Zone to the target backlight current.
ZONE BOUNDARY HYSTERESIS
For each Zone Boundary there are two Zone Boundary Registers: a Zone Boundary High Register and a Zone Boundary Low
Register (see Table 30). The difference between the Zone Boundary High and Zone Boundary Low Registers (for a specific zone)
creates the hysteresis that is required to transition between zones. This hysteresis prevents the backlight current from oscillating
between zones when the ALS voltage is close to a Zone Boundary Threshold. For Zone-to-Zone transitions the increasing ALS
voltage must cross the Zone Boundary High Threshold in order to get into the next higher zone. Conversely, the ALS decreasing
voltage must cross below the Zone Boundary Low Threshold in order to get into the next lower zone. Figure 6 describes this Zone
Boundary Hysteresis.
Copyright © 1999-2012, Texas Instruments Incorporated
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