LM3697_15 Datasheet, PDF (10/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
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7.3 Feature Descriptions
7.3.1 High-Voltage LED Control
7.3.1.1 High-Voltage Boost Converter
The high-voltage boost converter provides power for the three high-voltage current sinks (HVLED1, HVLED2,
and HVLED3). The boost circuit operates using a 4.7-µH to 22-µH inductor and a 1-µF output capacitor. The
selectable 500-kHz or 1-MHz switching frequency allows for use of small external components and provides for
high boost-converter efficiency. HVLED1, HVLED2, and HVLED3 feature an adaptive current regulation scheme
where the feedback point (HVLED1, HVLED2, and HVLED3) regulates the LED headroom voltage V_{HR_MIN}.
When there are different voltage requirements in the high-voltage LED strings (string mismatch), the LM3697
regulates the feedback point of the highest voltage string to V_{HR_MIN} and drop the excess voltage of the lower
voltage string across the lower strings current sink.
7.3.1.2 High-Voltage Current Sinks (HVLED1, HVLED2 and HVLED3)
HVLED1, HVLED2, and HVLED3 control the current in the high-voltage LED strings as configured by Control
Bank A or B. Each Control Bank has 5-bit full-scale current programmability and 11-bit brightness control.
Assignment of the high-voltage current sinks to control bank is done through the HVLED Current Sink Output
Configuration register (see Table 5).
7.3.1.3 High-Voltage Current String Biasing
Each high-voltage current string can be powered from the LM3697’s boost output (COUT) or from an external
source. The feedback enable bits (HVLED Current Sink Feedback Enables register bits [2:0]) determine where
the high-voltage current string anodes connect. 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 the LED headroom voltage V_{HR_MIN} at the current sink input.
When powered from alternate sources the feedback enable bits must 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 Characteristics for data on the high-voltage LED current vs V_{HR_MIN}).
Setting the HVLED Current Sink Feedback Enables register bits also determines triggering of the shorted high-
voltage LED String Fault flag (see the Fault Flags/Protection Features section).
7.3.2 Boost Switching-Frequency Select
The LM3697’s boost converter has two switching frequency settings. The switching frequency setting is
controlled via the Boost Frequency Select bit (bit 0 in the Boost Control register). Operating at the 500-kHz
switching frequency results in better efficiency under lighter load conditions due to the decreased switching
losses. In this mode the inductor must be between 10 µH and 22 µH. Operating at the 1-MHz switching
frequency results in better efficiency under higher load conditions resulting in lower conduction losses in the
MOSFETs and inductor. In this mode the inductor can be between 4.7 µH and 22 µH.
7.3.3 Automatic Switching Frequency Shift
The LM3697 has an automatic frequency select mode (bit 3 in the Boost Control register) to optimize the
frequency vs load dependent losses. In Auto-Frequency mode the boost converter switching frequency is
changed based on the high-voltage LED current. The threshold (Control A/B brightness code) at which the
frequency switchover occurs is configurable via the Auto-Frequency Threshold register. The Auto-Frequency
Threshold register contains an 8-bit code which is compared to the 8 MSB's of the brightness code. When the
brightness code is greater than the Auto-Frequency Threshold value the boost converter switching frequency is 1
MHz. When the brightness code is less than or equal to the Auto-Frequency Threshold register the boost
converter switching frequency is 500 kHz.
Figure 8 illustrates the LED efficiency improvement (3p5s LED configuration with a 4.7-µH inductor) when the
Auto-Frequency feature is enabled. When the LED brightness is less than or equal to 0x6C, the switching
frequency is 500 kHz, and it improves the LED efficiency by up to 6%. When the LED brightness is greater than
0x6C, the switching frequency is 1 MHz, and it improves LED efficiency by up to 2.2%.
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