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AME5142AAEEV Datasheet(PDF) 6 Page - Analog Microelectronics |
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AME5142AAEEV Datasheet(HTML) 6 Page - Analog Microelectronics |
6 / 18 page AME, Inc. 8 White LED Boost Converter In Tiny Package AME5142 Rev. A.01 n Detailed Description The AME5142 is a constant frequency step-up con- verter with an internal switch. The operations of AME5142 can be understood from block diagram clearly figure.2. The oscillator triggers the SET input of SR latch to turn on the power switch MS at the start of each cycle. A current sense voltage sum with a stabilizing ramp is con- nected to the positive terminal of the PWM comparator. When this voltage exceeds the output voltage of the error amplifier, the SR latch is reset to turn off the power switch till next cycle starts. The output voltage of the error am- plifier is amplified from the difference between the refer- ence voltage 0.15V and the feedback voltage. In this man- ner, if the error amplifiers voltage increases, more current is delivered to the output; if it decreases, less current is delivered. A 26V Zener diode connects from OVP pin to FB pin internally to provide an optional protection func- tion which prevents SW pin from over-voltage damage. Especially when the case of the feedback loop broken due to component wear-out or improper connection oc- curs. The behavior of OVP is to clamp the output voltage to 26V typically. This function is suitable for the applica- tions while driving white LEDs less than 6 in series. Current Limit Protection The AME5142 has current limiting protection to prevent excessive stress on itself and external components dur- ing overload conditions. The internal current limit com- parator will disable the NMOS power device at a typical switch peak current limit of 850mA. Output Over-Voltage Protection The AME5142 contains dedicated circuitry for monitor- ing the output voltage. In the event that the primary LED network is disconnected the output will increase and be limited to 26V (TYP), which will turn the NMOS off when the output voltage is at 26V (max.) until the output volt- age reach 26V (TYP.) or lower. The 26V limit allows the use of 26V 1 µF ceramic output capacitors creating an overall small solution for white LED applications. Under Voltage Protection The AME5142 has an UVP comparator to turn theNMOS power device off in case the input voltage or battery volt- age is too low preventing an on state of the power device conducting large amounts of current. Inductor Selection The recommended value of inductor for AME5142 ap- plications is 10 µH. Small size and better efficiency are the major concerns for portable device, such asAME5142 used for dual panel mobile phone. The inductor should have low DCR for better efficiency. To avoid inductor satu- ration, current rating should be at least 1A. The input range is 2.7V to 5.5V. Capacitor Selection 4.7 µF input capacitor can reduce input ripple. For bet- ter voltage stability, to increase the input capacitor value or using LC filter is feasible, especially in the Li-ion bat- tery application. 1 µF output capacitor is sufficient to re- duce output voltage ripple. For better voltage filtering, ceramic capacitors with low ESR are recommended. X5R and X7R types are suitable because of their wider volt- age and temperature ranges. Diode Selection Schottky diode is a good choice for AME5142 because of its lower forward voltage drop and faster reverse recov- ery. Using schottky diode can get better efficiency. The high speed rectification is also a good characteristic of schottky diode for high switching frequency. Current rat- ing of the diode must meet the root mean square of the peak current and output average current multiplication. Duty Cycle The maximum duty cycle of the switching regulator determines the maximum boost ratio of output-to- input voltage that the converter can attain in mode of operation. The duty cycle for a given boost application is defined as: This applies for continuous mode operation. V OUT + VDIODE - VIN V OUT + VDIODE - VSW D = n Application Hints |
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