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AAT3123AITP-20-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT3123AITP-20-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 18 page  AAT3123/23A/24 High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications 3123.2005.12.1.4 11 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 the level of V IN approaches V OUT. Refer to the Typical Characteristics section of this datasheet for meas- ured plots of efficiency versus input voltage and output load current for the given charge pump out- put 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) ceramic capacitors are used. In general, low ESR may be defined as less than 100m Ω. A value of 1µF for all four capacitors is a good starting point when choosing capacitors. If the LED current sources are only programmed for light current lev- els, then the capacitor size may be decreased. Capacitor Characteristics Ceramic composition capacitors are highly recom- mended over all other types of capacitors for use with the AAT3123/23A/24. Ceramic capacitors offer many advantages over their tantalum and alu- minum electrolytic counterparts. A ceramic capac- itor typically has very low ESR, is lowest cost, has a smaller PCB footprint, and is non-polarized. Low ESR ceramic capacitors help to maximize charge pump transient response. Since ceramic capaci- tors are non-polarized, they are not prone to incor- rect connection damage. Equivalent Series Resistance ESR is an important characteristic to consider when selecting a capacitor. ESR is a resistance internal to a capacitor that is caused by the leads, internal connections, size or area, material composition, and ambient temperature. Capacitor ESR is typi- cally measured in milliohms for ceramic capacitors and can range to more than several ohms for tanta- lum or aluminum electrolytic capacitors. Ceramic Capacitor Materials Ceramic capacitors less than 0.1µF are typically made from NPO or C0G materials. NPO and C0G materials typically have tight tolerance and are sta- ble over temperature. Large capacitor values are typically composed of X7R, X5R, Z5U, or Y5V dielectric materials. Large ceramic capacitors, typ- ically greater than 2.2µF, are often available in low- cost Y5V and Z5U dielectrics, but capacitors greater than 1µF are typically not required for AAT3123/23A/24 applications. Capacitor area is another contributor to ESR. Capacitors that are physically large will have a lower ESR when compared to an equivalent material smaller capacitor. These larger devices can improve circuit transient response when compared to an equal value capacitor in a smaller package size. Test Current/Channel Disable Each channel of the output is equipped with test current function. A small amount of current (~2µA) is injected into each output current source to detect the presence of load (LED). Unused channels that are tied to ground or LED load fail short will be automatically disabled instead of wasting the pro- grammed output current. The test current in the AAT3123A is higher (~150µA) to accommodate LEDs with lower impedance in failure mode. Thermal Protection The AAT3123/23A/24 has a thermal protection circuit that will shut down the internal LDO and charge pump if the die temperature rises above the thermal limit, as is the case during a short-circuit of the OUT pin. Driving Multiple LEDs, White LED Display Module Backlights, and Individual LEDs Connected in Parallel The AAT3123/23A/24 D1 to D6 outputs are true constant current sources capable of driving up to 20mA each over the operation input voltage range. Since these outputs are true constant current sources, they may be connected in parallel to drive a single power output. Any combination of outputs (D1 to D6) may be connected in parallel. The max- imum total output current is a sum of how many current sources are parallel connected. This fea- ture is particularly useful to power pre-manufac- |
Similar Part No. - AAT3123AITP-20-T1 |
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Similar Description - AAT3123AITP-20-T1 |
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