AAT3194

PRODUCT DATASHEET AAT3194 ChargePump TM High Efficiency 1.5X Fractional Charge Pump For White LED Applications Features • VIN Range: 2.7V to 5.5V • 20mA Full-Scale Current • Simple Serial Control (S2Cwire) Interface ▪ 32-Position Logarithmic Scale with Digital Control • Low Noise Constant Frequency Operation • 33% Less Input Current Than Doubler Charge Pump • High Accuracy Brightness Matching • Small Application Circuit • Regulated Output Current • Automatic Soft Start • No Inductors • 600kHz Switching Frequency • IQ 50ns 300ns < TLO < 75µs 50ns minimum to enable ON/1 2 3 4 5 6 n (n < =32) OFF Figure 3: Enable / Disable / LED Brightness Level Set Data Input. LED Selection The AAT3194 is designed to drive white LEDs with forward voltages to 4.2V. Since the D1:D4 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 AAT3194 uses a fractional charge pump. 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: internal switching losses and IC quiescent 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 the level of the power supply of IN approaches OUT. 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. Power Efficiency and Device Evaluation The charge pump efficiency discussion in the previous section only accounts for efficiency of the charge pump section itself. Due to the unique circuit architecture and design of the AAT3194, it is very difficult to measure efficiency in terms of a percent value comparing input power over output power. Since the device outputs are pure constant current sources, it is difficult to measure the output voltage for a given output (D1 to D4) to derive an 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 application should be based on trade-offs of size, external components count, reliability, operating range, and total energy 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= -or- POUT VOUT · IOUT V = = OUT PIN VIN · 1.5IOUT 1.5VIN η(%) = 100 ⎛ VOUT ⎞ ⎝ 1.5VIN⎠ For a charge pump with an output of 5V and a nominal input of 3.5V, the theoretical efficiency is 95%. Due to usage...not just “% efficiency.” 10 www.analogictech.com 3194.2008.05.1.1 PRODUCT DATASHEET AAT3194 ChargePump 700 600 TM High Efficiency 1.5X Fractional Charge Pump For White LED Applications 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 typically measured in milliohms for ceramic capacitors and can range to more than several ohms for tantalum or aluminum electrolytic capacitors. 40 60 80 100 VIN = 3.6V Input Power (mW) 500 400 300 200 100 0 0 20 Output (LED) Current (mA) Ceramic Capacitor Materials Ceramic capacitors less than 0.1µF are typically made from NPO or C0G materials. NPO and C0G materials generally have tight tolerance and are very stable over temperature. Larger capacitor values are usually composed of X7R, X5R, Z5U, or Y5V dielectric materials. Large ceramic capacitors (i.e., greater than 2.2µF) are often available in low-cost Y5V and Z5U dielectrics, but capacitors greater than 1µF are not typically required for AAT3194 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. Figure 3: Input Power vs. LED Current. Capacitor Selection Careful selection of the four external capacitors CIN, C1, C2, 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 LED current sources are only programmed for minimal 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 AAT3194. Ceramic capacitors offer many advantages over their tantalum and aluminum electrolytic counterparts. A ceramic capacitor typically has very low ESR, is lowest cost, has a smaller PCB footprint, and is nonpolarized. Low ESR ceramic capacitors help maximize charge pump transient response. Since ceramic capacitors are non-polarized, they are not prone to incorrect connection damage. Test Current/Channel Disable Each channel of the output is equipped with a test current function. The AAT3194 uses a small (~2µA) current source injected into each output pin to detect the presence of an LED. Unused channels other than channel 1 should be left open and will be automatically disabled instead of wasting the programmed output current. Thermal Protection The AAT3194 has a thermal protection circuit that will shut down the charge pump and current outputs if the die temperature rises above the thermal limit. 3194.2008.05.1.1 www.analogictech.com 11 PRODUCT DATASHEET AAT3194 AAT3194 ChargePump TM High Efficiency 1.5X Fractional Charge Pump For White LED Applications Ordering Information Package TSOPJW-12 Marking1 UUXYY Part Number (Tape and Reel)2 AAT3194ITP-20-T1 All AnalogicTech products are offered in Pb-free packaging. The term “Pb-free” means semiconductor products that are in compliance with current RoHS standards, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. For more information, please visit our website at http://www.analogictech.com/about/quality.aspx. Package Information TSOPJW-12 0.20 + 0.10 - 0.05 2.40 ± 0.10 0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC 2.85 ± 0.20 7° NOM 3.00 ± 0.10 0.9625 ± 0.0375 + 0.10 1.00 - 0.065 0.04 REF 0.055 ± 0.045 4° ± 4° 0.010 0.15 ± 0.05 0.45 ± 0.15 2.75 ± 0.25 All dimensions in millimeters. 1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD. Advanced Analogic Technologies, Inc. 3230 Scott Boulevard, Santa Clara, CA 95054 Phone (408) 737-4600 Fax (408) 737-4611 © Advanced Analogic Technologies, Inc. AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice. Except as provided in AnalogicTech’s terms and conditions of sale, AnalogicTech assumes no liability whatsoever, and AnalogicTech disclaims any express or implied warranty relating to the sale and/or use of AnalogicTech products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. In order to minimize risks associated with the customer’s applications, adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed. AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated. All other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders. 12 www.analogictech.com 3194.2008.05.1.1