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LM3535 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM3535 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 26 page  Circuit Description OVERVIEW The LM3535 is a white LED driver system based upon an adaptive 3/2× - 1× CMOS charge pump capable of supplying up to 200mA of total output current. With three separately controlled Groups of constant current sinks, the LM3535 is an ideal solution for platforms requiring a single white LED driver IC for main display, sub display, and indicator lighting. The tightly matched current sinks ensure uniform brightness from the LEDs across the entire small-format display. Each LED is configured in a common anode configuration, with the peak drive current set to 25mA. An I2C compatible interface is used to enable the device and vary the brightness within the individual current sink Groups. For GroupA , 128 exponentially-spaced analog brightness control levels are available. GroupB and GroupC have 8 linearly-spaced analog brightness levels. Additionally, the LM3535 provides 1 or 2 inputs (LM3535 has 1 and LM3535-2ALS has 2) for an Ambient Light Sensor to adaptively adjust the diode current based on ambient condi- tions, and a PWM pin to allow the diode current to be pulse width modulated to work with a display driver utilizing dynamic or content adjusted backlight control (DBC or CABC). CIRCUIT COMPONENTS Charge Pump The input to the 3/2× - 1× charge pump is connected to the V IN pin, and the regulated output of the charge pump is con- nected to the V OUT pin. The recommended input voltage range of the LM3535 is 2.7V to 5.5V. The device’s regulated charge pump has both open loop and closed loop modes of operation. When the device is in open loop, the voltage at V OUT is equal to the gain times the voltage at the input. When the device is in closed loop, the voltage at V OUT is regulated to 4.3V (typ.). The charge pump gain transitions are actively selected to maintain regulation based on LED forward voltage and load requirements. Diode Current Sinks Matched currents are ensured with the use of tightly matched internal devices and internal mismatch cancellation circuitry. There are eight regulated current sinks configurable into 3 different lighting regions. Ambient Light Sensing (ALS) and Interrupt The LM3535 provides an Ambient Light Sensing input (2 in- puts on LM3535-2ALS version) for use with ambient backlight control. By connecting the anode of a photo diode / sensor to the sensor input pins, and configuring the appropriate ALS resistors, the LM3535 or -2ALS version, can be configured to adjust the diode current to five unique settings, corresponding to four adjustable light region trip points. Additionally, when the LM3535 determines that an ambient condition has changed, the interrupt pin, when connected to a pull-up re- sistor will toggle to a '0' alerting the controller. See the I2C interface section for more details regarding the register con- figurations. Dynamic Backlight Control Input (PWM Pin) A PWM (Pulse Width Modulation) pin is provided on the LM3535 to allow a display driver utilizing dynamic backlight control (DBC), to adjust the LED brightness based on the content. The PWM input can be turned on or off (Acknowledge or Ignore) and the polarity can be flipped (active high or active low) through the I2C interface. The current sinks of the LM3535 require approximately 15µs. to reach steady-state target current. This turn-on time sets the minimum usable PWM pulse width for DBC/CABC. LED Forward Voltage Monitoring The LM3535 has the ability to switch gains (1x or 3/2x) based on the forward voltage of the LED load. This ability to switch gains maximizes efficiency for a given load. Forward voltage monitoring occurs on all diode pins. At higher input voltages, the LM3535 will operate in pass mode, allowing the V OUT voltage to track the input voltage. As the input voltage drops, the voltage on the Dxx pins will also drop (V DXX = VVOUT – V LEDx). Once any of the active Dxx pins reaches a voltage approximately equal to 130mV, the charge pump will switch to the gain of 3/2. This switch-over ensures that the current through the LEDs never becomes pinched off due to a lack of headroom across the current sinks. Once a gain transition occurs, the LM3535 will remain in the gain of 3/2 until an I2C write to the part occurs. At that time, the LM3535 will re-evaluate the LED conditions and select the appropri- ate gain. Only active Dxx pins will be monitored. Configurable Gain Transition Delay To optimize efficiency, the LM3535 has a user selectable gain transition delay that allows the part to ignore short duration input voltage drops. By default, the LM3535 will not change gains if the input voltage dip is shorter than 3 to 6 milliseconds. There are four selectable gain transition delay ranges (4 for LM3535-2ALS and 3 for LM3535) available on the LM3535. All delay ranges are set within the VF Monitor Delay Register . Please refer to the INTERNAL REGISTERS section of this datasheet for more information regarding the delay ranges. Hardware Enable (HWEN) The LM3535 has a hardware enable/reset pin (HWEN) that allows the device to be disabled by an external controller without requiring an I2C write command. Under normal oper- ation, the HWEN pin should be held high (logic '1') to prevent an unwanted reset. When the HWEN is driven low (logic '0'), all internal control registers reset to the default states and the part becomes disabled. Please see the Electrical Character- istics section of the datasheet for required voltage thresholds. I2C Compatible Interface DATA VALIDITY The data on SDIO line must be stable during the HIGH period of the clock signal (SCL). In other words, state of the data line can only be changed when SCL is LOW. 30082425 FIGURE 1. Data Validity Diagram A pull-up resistor between the controller's VIO line and SDIO must be greater than [ (VIO-V OL) / 3mA] to meet the VOL re- quirement on SDIO. Using a larger pull-up resistor results in lower switching current with slower edges, while using a 11 www.national.com |
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