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AAT3114ISN-20-T1 Datasheet(PDF) 9 Page - Advanced Analogic Technologies |
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AAT3114ISN-20-T1 Datasheet(HTML) 9 Page - Advanced Analogic Technologies |
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9 / 14 page  EN/SET Serial Interface The current source output magnitude is controlled by the EN/SET pin using AnalogicTech’s Simple Serial Control (S2Cwire) 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 mod- ulo 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 (see graph titled “IDIODE Response -9dB to -10dB” on page 5). 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 ini- tially sets the output LED current to -31dB, the low- est 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 out- put 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 (tSS). 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 tOFF timeout period. LED Selection The AAT3113/4 devices are designed to drive white LEDs with forward voltages of less than 4.2V. Since the D1:D6 output current sources are matched with negligible voltage dependence, the LED brightness will be matched regardless of their forward voltage matching. Charge Pump Efficiency The AAT3113/4 is a fractional charge pump. The efficiency ( η) can be simply defined as a linear volt- age regulator with an effective output voltage that is equal to one and a half times the input voltage. Efficiency ( η) for an ideal 1.5x charge pump can typically be expressed as the output power divided by the input power. η = POUT / PIN In addition, with an ideal 1.5x charge pump, the output current may be expressed as 2/3 of the input current. The expression to define the ideal efficiency ( η) can be rewritten as: η=POUT/PIN=(VOUTxIOUT)/(VINx1.5IOUT)=VOUT/1.5VIN η(%) = 100(VOUT / 1.5VIN) For a charge pump with an output of 5 volts and a nominal input of 3.5 volts, the theoretical efficiency is 95%. Due to internal switching losses and IC quies- cent current consumption, the actual efficiency can be measured at 93%. These figures are in close agreement for output load conditions from 1mA to 100mA. Efficiency will decrease as load current drops below 0.05mA or when level of VIN approach- es VOUT. Refer to the Typical Characteristics section for measured plots of efficiency versus input voltage and output load current for the given charge pump output voltage options. AAT3113/4 High Efficiency 1.5X Fractional Charge Pumps For White LED Applications 3113.2004.07.1.4 9 EN/SET Timing EN/SET Code 1 2 3 OFF OFF t HI t LO t OFF |
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