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AAT3164IRN-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT3164IRN-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 14 page  Additionally, a digital-to-analog converter can be used with the RSET pin to control the brightness level. The result is like the equation above, where VIO is replaced with VDAC: For cases where PWM dimming is preferred, the PWM signal can be applied directly to the RSET resis- tor, as shown in Figure 4. In order for the LED current to go to zero, the voltage level of the PWM signal must exceed the RSET pin voltage level of 0.6V. Figure 4: RSET Configuration for PWM Dimming Control. Device Power Efficiency The AAT3164 power conversion efficiency depends on the charge pump mode. By definition, device efficiency is expressed as the output power delivered to the LEDs divided by the total input power consumed. When the input voltage is sufficiently greater than the LED forward voltages, the device optimizes efficiency by operating in 1X mode. In 1X mode, the device is working as a bypass switch and pass- ing the input supply directly to the output. By sim- plifying the conditions such that the LEDs have uni- form VF, the power conversion efficiency can be approximated by: Due to the very low 1X mode quiescent current, the input current nearly equals the total output current delivered to the LEDs. Further, the low resistance bypass switch introduces negligible voltage drop from input to output. The AAT3164 further maintains optimized perform- ance and efficiency by detecting when the input voltage is not sufficient to sustain LED drive cur- rent. The device automatically switches to 1.5X mode when the input voltage drops too low in rela- tion to the LED forward voltages. In 1.5X mode, the output voltage can be boosted to 3/2 the input voltage. The 3/2 conversion ratio introduces a corresponding 1/2 increase in input current. For ideal conversion, the 1.5X mode effi- ciency is given by: Similarly, when the input falls further, such that 1.5X mode can no longer sustain LED drive cur- rent, the device will automatically switch to 2X mode. In 2X mode, the output voltage can be boosted to twice the input voltage. The doubling conversion ratio introduces a corresponding dou- bling of the input current. For ideal conversion, the 2X mode efficiency is given by: Additional Applications The current sinks of the AAT3164 can be combined to drive higher current levels through a single LED. As an example, each LED of a three-die arrange- ment can be driven at 80mA each, or 240mA total (see Figure 5). Other applications that require constant current can also be accommodated by combining constant current channels to achieve the necessary current levels. η = ≈ V F · ILED V IN · 2IIN V F 2 · V IN η = ≈ V F · ILED V IN · 1.5IIN V F 1.5 · V IN η = ≈ V F · ILED V IN · IIN V F V IN η = P OUT P IN AAT3164 RSET R1 PWM I LED(LO) = 870 - ⎛⎞ ⎝⎠ 0.6 R 1 // R2 V DAC R 2 AAT3164 High Efficiency 1X/1.5X/2X Charge Pump for Six-Channel White LED Applications 3164.2006.07.1.0 11 |
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