Abstract: This switch-mode power supply provides ±15V at 0.5A output from a 4.5V to 12V input-voltage range. The wide input-voltage range allows this power supply the flexibility to be powered from a regulated DC voltage, or even an unregulated DC voltage such as the rectified output from an inexpensive AC step-down transformer. The power supply consists of a MAX668 boost circuit and a MAX1846 inverting circuit. Each circuit operates at a 300kHz switching frequency as a compromise among cost, size, and performance. Both circuits limit the maximum switch current, which ultimately limits the output current for a given input voltage. However, the maximum output current increases as the input voltage increases. The MAX668 circuit in Figure 1 contains a few components in addition to the minimum circuit implementation. C7 adds a pole to compensate for the ESR-zero of the output capacitor. R5 and C8 filter the current-sense voltage to prevent high-frequency switching noise from tripping the current limit. This is in addition to the internal 60ns current-sense blanking time. Figure 1. MAX668 Boost circuit for +15V at 0.5A. The MAX668 output voltage can be changed to +12V by changing R2 to 86.6kΩ. The minimum input voltage then decreases to approximately 10V. No other changes are required because the MAX668 has internal compensation. The output ripple voltage caused by switching can be reduced an order of magnitude with a secondary output filter set to one-tenth of the switching frequency. A 1Ω, 0.5W resistor placed in series with a 10µF 25V capacitor with less than 100mΩ ESR is sufficient. This introduces an additional 0.5V decrease in output voltage for a 0.5A load. The feedback voltage must be sensed before the secondary filter for the MAX668 to work properly. The MAX1846 circuit (Figure 2) also contains a few components in addition to the minimum circuit implementation. Like the C7 for the MAX668, C20 adds a pole to compensate for the ESR-zero of the output capacitor. As with the MAX668, R16 and C22 filter the current-sense voltage to prevent high-frequency switching noise from tripping the current limit. This is in addition to the internal 100ns current-sense blanking time. The MAX1846 EXT pin has a controlled slew rate that inherently limits the high-frequency switching noise. Table 1. MAX668 Application Circuit Capabilities VIN IIN VOUT IOUT Efficiency 5.00 0.0007 15.11 0   5.00 1.597 14.81 0.50 0.927 6.00 1.318 14.85 0.50 0.939 8.00 0.981 14.91 0.50 0.950 10.00 0.781 14.96 0.50 0.958 11.00 0.708 14.98 0.50 0.962 12.00 0.648 15.01 0.50 0.965 4.47 1.800 14.78 0.50 0.918 20MHz BW   240mVP-P     4.46 2.556 14.70 0.70 0.903 Current limit     ~0.71   BILL OF MATERIALS 4.5V to 10V Input 15V at 0.5A Output 668_15g DESIGNATION QTY DESCRIPTION C1 1 10µF, 25V X5R ceramic capacitor (1210) Taiyo Yuden TMK325BJ106MM C2 1 0.22µF, 25V ceramic capacitor (0805) Taiyo Yuden UMK212BJ224MG C3 1 0.22µF, 10V ceramic capacitor (0603) Taiyo Yuden EMK107BJ224MA C4 1 1µF, 10V X5R ceramic capacitor (0603) Taiyo Yuden LMK107BJ105MA C5 1 390µF, 25V aluminum electrolytic capacitor Sanyo 25MV390AX C6 1 1µF, 25V X5R ceramic capacitor (1206) C7 1 2.2nF ceramic capacitor (0603) C8 1 1nF ceramic capacitor (0603) D1 1 3A, 60V Schottky diode Nihon EC31QS06 L1 1 22µH, 6A power inductor Coilcraft DO5022P-223 N1 1 30mΩ, 30V n-channel MOSFET (SO-8) Fairchild FDS6612A R1 1 0.033Ω, 0.5W, 5% resistor (2012) R2 1 110kΩ, 1% resistor (0603) R3 1 10.0kΩ, 1% resistor (0603) R4 1 165kΩ, 1% resistor (0603) R5 1 100Ω, 5% resistor (0603) U1 1 MAX668EUB (10-µMAX®) Figure 2. MAX1846 circuit for -15V at 0.5A. The MAX1846 output voltage can be changed to -12V by changing R13 to 97.6kΩ and R10 to 91kΩ. The maximum input voltage does not decrease, though duty cycle jitter increases somewhere between 10V and 12V input. Again, the output ripple voltage caused by switching can be reduced an order of magnitude with the same secondary filter described for the MAX668. The feedback voltage must be sensed before the secondary filter for the MAX1846 to work properly. Table 2. MAX1846 Application Circuit Capabilities VIN IIN VOUT IOUT Efficiency 5.00 0.0077 -15.15 0   5.00 1.76 -15.15 0.50 0.861 6.01 1.42 -15.15 0.50 0.888 8.01 1.05 -15.15 0.50 0.901 10.00 0.84 -15.15 0.50 0.902 12.00 0.70 -15.15 0.50 0.902 4.48 2.04 -15.15 0.50 0.829 20MHz BW   360mVP-P     4.48 3.63 -15.15 0.80 0.745 Current limit     ~0.81   BILL OF MATERIALS 4.5V to 12V Input -15V at 0.5A Output 1846-15d DESIGNATION QTY DESCRIPTION C11 1 0.47µF ceramic capacitor (0603) C12,C13 2 10µF, 25V X5R ceramic capacitor (1210) Taiyo Yuden TMK325BJ106MM C15 1 68nF ceramic capacitor (0603) C16 1 100pF ceramic capacitor (0603) C17 1 390µF, 25V aluminum electrolytic capacitor Sanyo 25MV390AX C18 1 1µF, 25V X5R ceramic capacitor (1206) C19 1 0.1µF ceramic capacitor (0603) C20 1 2.2nF ceramic capacitor (0603) C22 1 1nF ceramic capacitor (0603) D2 1 5A, 40V Schottky diode Central Semi CMSH5-40 L2 1 33µH, 5A inductorCoilcraft DS5022P-333 P2 1 35mΩ, -30V p-channel MOSFET (SO-8) Fairchild FDS6685 R9 1 22kΩ, 5% resistor (0603) R10 1 110kΩ, 5% resistor (0603) R11 1 150kΩ, 5% resistor (0603) R12 1 0.02Ω, 1W, 1% resistor (2512) Dale WSL-2512-R020-F R13 1 121kΩ, 1% resistor (0603) R14 1 10.0kΩ, 1% resistor (0603) R16 1 100Ω, 5% resistor (0603) U2 1 MAX1846EUB (10-µMAX) 相关型号   APP 3843: May 23, 2006 MAX1846 高效率、电流模式、反相PWM控制器 完整的数据资料 (PDF, 352kB) 免费样品 MAX668 1.8V至28V输入、PWM升压型控制器，µMAX封装 完整的数据资料 (PDF, 304kB) 免费样品 自动更新 需要自动接收最新发布的应用笔记吗？请订阅EE-Mail™ (English only)。 我们期待您的反馈！ 喜欢？不喜欢？有待改善？或为我们提供建议？请与我们联系 — 我们将根据您的意见或建议改善我们的工作。 网页评价或提供建议   下载，PDF格式 (52kB)  AN3843, AN 3843, APP3843, Appnote3843, Appnote 3843