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AAT3120 Datasheet(PDF) 8 Page - Advanced Analogic Technologies |
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AAT3120 Datasheet(HTML) 8 Page - Advanced Analogic Technologies |
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8 / 14 page  AAT3120 3-Channel Charge Pump For White LED Applications 8 3120.2005.11.1.1 Table 1: Current Level Settings. Constant Current Output Level Settings The constant current source output amplitude for output D1 to D3 is set via the serial interface. Because the outputs D1 to D3 are true independent constant current sources, the voltage observed on any single given output will be determined by the actual forward voltage (V F) for the LED being driven. Since the output current of the AAT3120 is pro- grammable through its simple single-wire serial interface, no PWM (pulse width modulation) or additional control circuitry is needed to control LED brightness. This feature greatly reduces the burden on a microcontroller or system IC to manage LED or display brightness, allowing the user to "set it and forget it." EN/SET Serial Interface The current source output magnitude is controlled by the EN/SET serial data single-wire interface. The interface records rising edges of the EN/SET pin and decodes them into eight individual current level settings. Code 8 is full scale. 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 zero. The next lowest setting is 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 accommo- date a wide range of data rates. After the first rising edge of EN/SET, the charge pump is enabled and reaches 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 the 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. Test Current/Channel Disable Each channel of the output is equipped with a test current function. A small amount of current (~2µA) is injected into each output current source to detect the presence of load (LED). For unused channels that are tied to ground or load LED fail short, those channels will be automatically disabled instead of wasting the programmed output current. Code I OUT, Each LED (mA) 10 2 0.5 3 1.3 4 3.3 5 8.3 6 11.1 715 820 EN/SET Timing EN/SET Code 1 2 3 OFF OFF t HI t LO t OFF |
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