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AAT3124 Datasheet(PDF) 9 Page - Advanced Analogic Technologies |
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AAT3124 Datasheet(HTML) 9 Page - Advanced Analogic Technologies |
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9 / 18 page  AAT3123/23A/24 High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications 3123.2005.12.1.4 9 EN/SET Serial Interface The current source output magnitude is controlled by the EN/SET pin, using AnalogicTech’s Simple Serial Control (S 2Cwire) interface. The interface records rising edges of the EN/SET pin, and decodes them into 32 individual current level set- tings each 1dB apart. Code 32 is full scale, and Code 1 is full scale attenuated by 31dB. The modu- lo 32 interface wraps states back to State 1 after the 32nd clock, so 1dB of attenuation is achieved by clocking the EN/SET pin 31 times. The counter can be clocked at speeds up to 1MHz, so intermediate states are not visible. The first rising edge of EN/SET enables the IC and initially sets the output LED current to -31dB, the lowest setting equal to 525µA. Once the final clock cycle is input for the desired brightness level, the EN/SET pin is held high to maintain the device output current at the programmed level. The device is disabled 500µs after the EN/SET pin transitions to a logic low state. The EN/SET timing is designed to accommodate a wide range of data rates. After the first rising edge of EN/SET, the charge pump is enabled and reach- es full capacity after the soft-start time (T SS). During the soft-start time, multiple clock pulses may be entered on the EN/SET pin to set the final output current level with a single burst of clocks. Alternatively, the EN/SET clock pulses may be entered one at a time to gradually increase LED brightness over any desired time period. A constant current is sourced as long as EN/SET remains in a logic high state. The current source outputs are switched off after EN/SET has remained in a low state for at least the t OFF timeout period. EN/SET Timing EN/SET Code 1 2 3 OFF OFF t HI t LO t OFF LED Selection The AAT3123/23A/24 is specifically intended for driv- ing white LEDs. However, the device design will allow the AAT3123/23A/24 to drive most types of LEDs with forward voltage specifications ranging from 2.0V to 4.3V. LED applications may include main and sub-LCD display backlighting, camera photo-flash applications, color (RGB) LEDs, infrared (IR) diodes for remotes, and other loads benefiting from a controlled output current generated from a varying input voltage. Since the D1 to D6 output cur- rent sources are matched with negligible voltage dependence, LED brightness will be matched regardless of the specific LED forward voltage (V F) levels. In some instances (e.g., in high luminous output applications such as photo flash), it may be neces- sary to drive high-V F type LEDs. The low-dropout current sources in the AAT3123/23A/24 make it capable of driving LEDs with forward voltages as high as 4.3V at full current from an input supply as low as 3.0V. Outputs can be paralleled to drive high-current LEDs without complication. Device Switching Noise Performance The AAT3123/23A/24 operates at a fixed frequency of approximately 1MHz to control noise and limit harmonics that can interfere with the RF operation of cellular telephone handsets or other communica- tion devices. Back-injected noise appearing on the input pin of the charge pump is 20mV peak-to-peak, typically ten times less than inductor-based DC/DC boost converter white LED backlight solutions. The AAT3123/23A/24 soft-start feature prevents noise transient effects associated with inrush currents during start-up of the charge pump circuit. |
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