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AAT3104 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT3104 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 15 page  AAT3104178 Low Cost 1x/2x 4 Channel Charge Pump WLED Driver ChargePumpTM PRODUCT DATASHEET 3104.2008.09.1.0 11 www .analogictech.com to outputs D1 through for a given constant current set- ting. Depending on the combination of VIN and voltages sensed at the current sources, the device will operate in load switch mode. When any one of the voltages sensed at the current sources nears dropout, the device will operate in 2X charge pump mode. Each of these modes will yield different efficiency values. Refer to the following two sections for explanations for each operational mode. 1X Mode Efficiency The AAT3104 1X mode is operational at all times and functions alone to enhance device power conversion effi- ciency when VIN is higher than the voltage across the load. When in 1X mode, voltage conversion efficiency is defined as output power divided by input power. An expression for the ideal efficiency (η) in 1X charge- pump mode can be expressed as: P OUT P IN V F · ILED V IN · IIN V F · ILED V IN · IOUT V F V IN η = = = ≈ -or- V F V IN η (%) = · 100 For a charge pump led driver with VF of 3.2V and 4.2V input voltage, the theoretical efficiency is 76%. Due to internal switching losses and IC quiescent current con- sumption, the actual efficiency can be measured at 73%. 2X Charge Pump Mode Efficiency The AAT3104 contains a charge pump which will boost the input supply voltage in the event where VIN is less than the voltage required to supply the output. The effi- ciency (η) can be simply defined as a linear voltage regulator with an effective output voltage that is equal to one and two times the input voltage. Efficiency (η) for an ideal 2X charge pump can typically be expressed as the output power divided by the input power. η = P F P IN In addition, with an ideal 2X charge pump, the output current may be expressed as 1/3 of the input current. The expression to define the ideal efficiency (η) can be rewritten as P OUT P IN V F · ILED V IN · IIN V F · ILED V IN · 2 · IOUT V F 2 · V IN η = = = ≈ -or- V F 2 · V IN η (%) = · 100 For a charge pump current source driver with VF of 3.2V and 2.7V input voltage, the theoretical efficiency is 59%. Due to internal switching losses and IC quiescent current consumption, the actual efficiency can be measured at 57%. Efficiency will decrease substantially as load cur- rent drops below 1mA or when the voltage level at VIN approaches the voltage level at VOUT. Additional Applications The current sources of the AAT3104 can be combined freely to drive higher current levels through one LED. As an example, a single LED can be driven at 120mA by combining together D1 through D4 outputs. For lower-cost applications, the flying capacitor can be removed; C+ and C- should be floating. This will force AAT3104 to operate in 1X mode. To maintain regulated LED current, the input supply voltage has to be higher than the charge-pump's dropout voltage in 1X mode plus the forward voltage of the LED at the preset LED current. Capacitor Selection Careful selection of the three external capacitors CIN, CP, and COUT is important because they will affect turn-on time, output ripple, and transient performance. Optimum performance will be obtained when low equivalent series resistance (ESR) ceramic capacitors are used, in general, low ESR may be defined as less than 100mΩ. A value of 1μF for all four capacitors is a good starting point when choosing capacitors. If the constant current sources are only programmed for light current levels, then the capacitor size may be decreased. Capacitor Characteristics Ceramic composition capacitors are highly recommended over all other types of capacitors for use with the AAT3104. Ceramic capacitors offer many advantages over their tantalum and aluminum electrolytic counter- parts. A ceramic capacitor typically has very low ESR, is lowest cost, has a smaller PCB footprint, and is non- polarized. Low ESR ceramic capacitors help maximizing charge pump transient response. Since ceramic capaci- tors are non-polarized, they are not prone to incorrect connection damage. |
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