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AAT3151B Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT3151B Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
11 / 13 page AAT3151B High Efficiency 1X/1.5X Charge Pump For White LED Applications ChargePumpTM PRODUCT DATASHEET 3151B.2008.02.1.1 11 www .analogictech.com Applications Information LED Selection The AAT3151B is specifically intended for driving white LEDs. However, the device design will allow the AAT3151B to drive most types of LEDs with forward voltage speci- fications ranging from 2.0V to 4.3V. LED applications may include main and sub-LCD display backlighting, camera photo-flash applications, color (RGB) LEDs, infrared (IR) diodes for remotes, and other loads bene- fiting from a regulated output current generated from a varying input voltage. Since the D1 to D4 constant cur- rent sinks are matched with negligible voltage depen- dence, the LED brightness will be matched regardless of the specific LED forward voltage (VF) levels. In some instances (e.g., in high luminous output appli- cations such as photo flash), it may be necessary to drive high-VF type LEDs. The low dropout current sinks in the AAT3151B make it capable of driving LEDs with forward voltages as high as 4.3V at full current from an input supply as low as 3.0V. Current sinks can be paral- leled to drive high-current LEDs without complication. Device Switching Noise Performance The AAT3151B operates at a fixed frequency of approxi- mately 1MHz to control noise and limit harmonics that can interfere with the RF operation of cellular telephone handsets or other communication devices. Back-injected noise appearing on the input pin of the charge pump is 20mV peak-to-peak, typically ten times less than induc- tor-based DC/DC boost converter white LED backlight solutions. The AAT3151B soft-start feature prevents noise transient effects associated with inrush currents during start-up of the charge pump circuit. Power Efficiency and Device Evaluation The charge pump efficiency discussion in the following sections only accounts for the efficiency of the charge pump section itself. Due to the unique circuit architec- ture and design of the AAT3151B, it is very difficult to measure efficiency in terms of a percent value compar- ing input power over output power. Since the AAT3151B inputs are pure constant current sinks and typically drive individual loads, it is difficult to measure the output voltage for a given input (D1 to D4) to derive an overall output power measurement. For any given application, white LED forward voltage levels can differ, yet the load 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 component count, reliability, operating range, and total energy usage...not just % efficiency. The AAT3151B efficiency may be quantified under very specific conditions and is dependent upon the input volt- age versus the output voltage seen across the loads applied to inputs D1 through D4 for a given constant current setting. Depending on the combination of VIN and voltages sensed at the current sinks, the device will operate in load switch mode. When any one of the volt- ages sensed at the current sinks nears dropout, the device will operate in 1.5X charge pump mode. Each of these modes will yield different efficiency values. Refer to the following two sections for explanations for each operational mode. Load Switch Mode Efficiency The AAT3151B load switch mode is operational at all times and functions alone to enhance device power con- version efficiency when VIN is greater than the voltage across the load. When in load switch mode, the voltage conversion efficiency is defined as output power divided by input power: η = P OUT P IN The expression to define the ideal efficiency (η) can be rewritten as: η = P OUT = V OUT · IOUT = V OUT P IN V IN · IOUT V IN -or- η(%) = 100 V OUT V IN ⎛⎞ ⎝⎠ |
Similar Part No. - AAT3151B_08 |
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Similar Description - AAT3151B_08 |
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