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ACE701435BM+ Datasheet(PDF) 5 Page - ACE Technology Co., LTD. |
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ACE701435BM+ Datasheet(HTML) 5 Page - ACE Technology Co., LTD. |
5 / 13 page ACE701 Technology PFM STEP-UP DC/DC Converter with High Efficiency and Low Noise VER 1.2 5 High-gain differential error amplifier guarantees stable output voltage at difference input voltage and load. In order to reduce ripple and noise, the error amplifier is designed with high band-with. Though at very low load condition, the quiescent current of chip do effect efficiency certainly. The four main energy loss of Boost structure DC-DC converter in full load are the ESR of inductor, the voltage of Schottky diode, on resistor of internal N-channel MOSFET and its driver. In order to improve the efficiency, ACE701 integrates low on-resistor N-channel MOSFET and well design driver circuits. The switch energy loss is limited at very low level. Selection of the External Components Thus it can be seen, the inductor and schottky diode affect the conversion efficiency greatly. The inductor and the capacitor also have great influence on the output voltage ripple of the converter. So it is necessary to choose a suitable inductor, a capacitor and a right schottky diode, to obtain high efficiency, low ripple and low noise. Before discussion,we define D≣Vout-Vin / Vout (1)Inductor Selection Above all, we should define the minimum value of the inductor that can ensure the boost DC-DC to operate in the continuous current-mode condition. Lmin≧D(1-D)2RL / 2f The above expression is got under conditions of continuous current mode, neglect Schottky diode’s voltage, ESR of both inductor and capacitor. The actual value is greater that it. If inductor’s value is less than Lmin,the efficiency of DC-DC converter will drop greatly, and the DC-DC circuit will not be stable. Secondly, consider the ripple of the output voltage, Δ I=D‧Vin / Lf Im ax=Vin / (1-D)2RL + DVin / 2Lf If inductor value is too small, the current ripple through it will be great. Then the current through diode and power switch will be great. Because the power switch on chip is not ideal switch, the energy of switch will improve. The efficiency will fall. Thirdly,in general, smaller inductor values supply more output current while larger values start up with lower input voltage and acquire high efficiency. An inductor value of 3uH to 1mH works well in most applications. If DC-DC converter delivers large output current (for example: output current is great than 50mA), large inductor value is recommended in order to improve efficiency. If DC-DC must output very large current at low input supply voltage, small inductor value is recommended. The ESR of inductor will effect efficiency greatly. Suppose ESR value of inductor is rL,Rload is load resistor,then the energy can be calculated by following expression: Δη≈ RL / Rload (1-D) 2 |
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