LM3533 Datasheet, PDF (16/53 Pages) Texas Instruments – Complete Lighting Power Solution for Smartphone Handsets

LM3533
FIGURE 3. Functional Control Diagram
30135739
High-Voltage Boost Converter
The high-voltage boost converter provides power for the two high-voltage current sinks (HVLED1 and HVLED2). The boost circuit
operates using a 4.7µH to 22µH inductor and a 1µF output capacitor. The selectable 500kHz or 1MHz switching frequency allows
for the use of small external components and provides for high boost-converter efficiency. Both HVLED1 and HVLED2 feature an
adaptive current regulation scheme where the feedback point (HVLED1 or HVLED2) is regulated to a minimum of 400mV. When
there are different voltage requirements in both high-voltage LED strings (string mismatch), the LM3533 will regulate the feedback
point of the highest voltage string to 400mV and drop the excess voltage of the lower voltage string across the lower strings current
sink.
HIGH-VOLTAGE CURRENT SINKS (HVLED1 and HVLED2)
HVLED1 and HVLED2 control the current in the high-voltage LED strings. Each current sink has 5-bit full-scale current pro-
grammability and 8-bit brightness control. Either current sink can have its current set through a dedicated brightness register or be
controlled via the ambient light sensor interface. Configuration of the high-voltage current sinks is done through the Control A/B
Brightness Configuration Register (see Table 8).
HIGH-VOLTAGE CURRENT STRING BIASING
Each high-voltage current string can be powered from the LM3533’s boost output (COUT) or from an external source. The Anode
Connect Register bits [1:0] determine where the high-voltage current string anodes will be connected. When set to '1' (default) the
high-voltage current sink inputs are included in the boost feedback loop. This allows the boost converter to adjust its output voltage
in order to maintain at least 400mV at the current sink input.
When powered from alternate sources, bits [1:0] should be set to '0'. This removes the particular current sink from the boost feedback
loop. In these configurations the application must ensure that the headroom voltage across the high-voltage current sink is high
enough to prevent the current sink from going into dropout (see the Typical Performance Characteristics for data on the high-voltage
LED current vs headroom voltage).
Setting the Anode Connect Register bits also determines how the shorted high-voltage LED String Fault flag is triggered (see Fault
Flags/Protection Features section).
BOOST SWITCHING-FREQUENCY SELECT
The LM3533’s boost converter can have a 1MHz or 500kHz switching frequency. For a 500kHz switching frequency the inductor
must be between 10µH and 22µH. For the 1MHz switching frequency the inductor can be between 4.7µH and 22µH. The boost
frequency is programmed through bit [1] of the OVP/Boost Frequency/PWM Polarity Select register.
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