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AAT3132AITP-1-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT3132AITP-1-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 14 page  AAT3132/32A High Efficiency 1.5X Fractional Charge Pump for White LED Applications 3132.2005.12.1.4 11 Power Efficiency and Device Evaluation The charge pump efficiency discussion in the fol- lowing sections only accounts for efficiency of the charge pump section itself. Due to the unique circuit architecture and design of the AAT3132/32A, it is very difficult to measure efficiency in terms of a per- cent value comparing input power over output power. Since the AAT3132/32A 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 D4) 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 components count, reliability, operating range, and total energy usage...not just % efficiency. The AAT3132/32A efficiency may be quantified under very specific conditions and is dependent upon the input voltage versus the output voltage seen across the loads applied to outputs D1 through D4 for a given constant current setting. The efficiency ( η) can be simply defined as a linear voltage regulator with an effective output voltage that is equal to one and one 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. 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: -or- For a charge pump with an output of 5V and a nom- inal input of 3.5V, the theoretical efficiency is 95%. Due to internal switching losses and IC quiescent current consumption, the actual efficiency can be measured at 93%. These figures are in close agree- ment for output load conditions from 1mA to 100mA. Efficiency will decrease as load current drops below 0.05mA or when level of V IN approaches VOUT. Refer to the Typical Characteristics section of this datasheet for measured plots of efficiency versus input voltage and output load current for the given charge pump output voltage options. Capacitor Selection Careful selection of the four external capacitors C IN, C1, C2, and COUT is important because they will affect turn-on time, output ripple, and transient per- formance. Optimum performance will be obtained when low equivalent series resistance (ESR) η(%) = 100 V OUT 1.5V IN ⎛⎞ ⎝⎠ η = P OUT = V OUT × I OUT = V OUT P IN V IN × 1.5I OUT 1.5V IN η = P OUT P IN EN/SET Timing Diagram EN/SET Code 1 2 3 OFF OFF t HI t LO t OFF |
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