 전자부품 반도체 검색엔진( 무료 PDF 다운로드 ) - 데이터시트뱅크
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# MAX774CPA 데이터 시트보기 (PDF) - Maxim Integrated

 부품명 상세내역 제조사 MAX774CPA -15V or adjustable, high-efficiency, low IQ inverting DC-DC controller Maxim Integrated MAX774CPA Datasheet PDF : 16 Pages
 First Prev 11 12 13 14 15 16 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers 2500 RSENSE = 0.05Ω 2000 RSENSE = 0.06Ω 1500 1000 RSENSE = 0.07Ω RSENSE = 0.08Ω 500 RSENSE = 0.09Ω VOUT = -5V 0 3 4 5 6 7 8 9 10 11 12 13 14 15 INPUT VOLTAGE (V) Figure 6. MAX774 Maximum Output Current vs. Input Voltage (VOUT = -5V) 1000 800 RSENSE = 0.05Ω RSENSE = 0.06Ω 600 RSENSE = 0.07Ω VOUT = -12V 400 200 RSENSE = 0.08Ω RSENSE = 0.09Ω 0 3 4 5 6 7 8 9 INPUT VOLTAGE (V) Figure 7. MAX775 Maximum Output Current vs. Input Voltage (VOUT = -12V) Choosing an Inductor Practical inductor values range from 10µH to 50µH. The maximum inductor value is not particularly critical. For highest current at high VOUT to V+ ratios, the inductor should not be so large that the peak current never reaches the current limit. That is: L(max) ≤ __[_V_+_(_m__i_n_)_-__V_S_W__(_m_a__x_)_]_x__1_2_µ__s_ ILIM(max) This is only important if __V_IN____ < 1 — = _t_O__F_F_(m__i_n_)_ VOUT 6 tON(max) More important is that the inductor not be so small that the current rises much faster than the current-limit comparator can respond. This would be wasteful and reduce effi- ciency. Calculate the minimum inductor value as follows: L(min) ≥ _[_V__+_(_m__a_x_)_-__V_S_W__(_m__in__)]__x_0__.3_µ__s_ δ(I) x ILIM(min) Where L is in µH, 0.3µs is an ample time for the com- parator response, ILIM is the current limit (see Current- Sense Resistor section), and δ(I) is the allowable per- centage of overshoot. As an example, Figure 2's circuit uses a 3A peak current. If we allow a 15% overshoot and 15V is the maximum input voltage, then L(min) is 16µH. The actual value of L above this limit has minimal effect on this circuit's operation. For highest efficiency, use a coil with low DC resistance. Coils with 30mΩ or lower resistance are available. To minimize radiated noise, use a torroid, pot-core, or shield- ed-bobbin inductor. Inductors with a ferrite core or equiv- alent are recommended. Make sure that the inductor’s saturation current rating is greater than ILIM(max). Diode Selection The ICs’ high switching frequencies demand a high- speed rectifier. Schottky diodes such as the 1N5817 to 1N5822 families are recommended. Choose a diode with an average current rating approximately equal to or greater than ILIM (max) and a voltage rating higher than VIN(max) + VOUT. For high-temperature applica- tions, Schottky diodes may be inadequate due to their high leakage currents; instead, high-speed silicon diodes may be used. At heavy loads and high temper- ature, the benefits of a Schottky diode’s low forward voltage may outweigh the disadvantages of its high leakage current. Current-Sense Resistor The current-sense resistor limits the peak switch cur- rent to 210mV/RSENSE, where RSENSE is the value of the current-sense resistor, and 210mV is the current- sense comparator threshold (see Current-Limit Trip Level in the Electrical Characteristics). To maximize efficiency and reduce the size and cost of external components, minimize the peak current. However, since the output current is a function of the peak current, do not set the limit too low. Refer to Figures 6–9 to determine the sense resistor (and, there- fore, peak current) for the required load current. 12 ______________________________________________________________________________________