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AAT3123ITP-20-T1 Datasheet(PDF) 10 Page - List of Unclassifed Manufacturers |
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AAT3123ITP-20-T1 Datasheet(HTML) 10 Page - List of Unclassifed Manufacturers |
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10 / 18 page  AAT3123/4 High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications 10 3123.2003.09.1.1 Power Efficiency and Device Evaluation The charge pump efficiency discussion in the fol- lowing sections only account for the efficiency of the charge pump section itself. Due to the unique circuit architecture and design of the AAT3123/24, it is very difficult to measure efficiency in terms of a percent value comparing input power over output power. Since the AAT3123/24 outputs are pure constant current sources and typically drive individual loads, it is difficult to measure the output voltage for a given output (D1 to D6) to derive an overall output power measurement. For any given application, white LED forward voltage levels can differ, yet the output drive current will be maintained as a constant. This makes quantifying output power a difficult task when taken in the context of comparing to other white LED driver circuit topologies. A better way to quantify total device efficiency is to observe the total input power to the device for a given LED current drive level. The best White LED driver for a given applica- tion should be based on trade-offs of size, external component count, reliability, operating range and total energy usage...Not just "% efficiency". The AAT3123/24 efficiency may be quantified under very specific conditions and is dependant upon the input voltage versus the output voltage seen across the loads applied to outputs D1 through D4 or D6 for a given constant current set- ting. Depending upon the case of VIN being greater than the specific voltage seen across the load on D1 (or D5 when the AAT3124 is used) the device will operate in "Load Switch" mode. If the voltage seen on the constant current source output is less than VIN then the device will operate in 1.5X charge pump mode. Each of these two modes will yield different efficiency values. One should refer to the following two sections for explanations for each operational mode. Load Switch Mode Efficiency The AAT3123/24 load switch mode is operational at all times and functions alone to enhance device power conversion efficiency when the condition exists where VIN is greater then voltage across the load connected to the constant current source out- puts. When in "Load Switch" mode, the voltage conversion efficiency is defined as output power divided by input power: η = POUT/PIN The expression to define the ideal efficiency ( ) can be rewritten as: η = POUT/PIN = (VOUT x IOUT)/(VIN x IOUT) = VOUT/VIN or η(%) = 100(VOUT/VIN) Charge Pump Section Efficiency The AAT3123/24 contains a fractional charge pump which will boost the input supply voltage in the event where VIN is less then the voltage required on the constant current source outputs. The effi- ciency ( η) can be simply defined as a linear voltage 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 = (VOUT x IOUT)/(VIN x 1.5IOUT) = 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 quiescent current consumption, the actual efficien- cy 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 approaches VOUT. Refer to the Typical Character- istics section for measured plots of efficiency ver- sus input voltage and output load current for the given charge pump output voltage options. |
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