Abstract: A MAX8538 dual synchronous buck controller supplies both +5V and -5V outputs at 3A. One controller operates as a synchronous recitifed step-down (buck) converter while the second controller runs a transformer driven inverter. The circuit also features soft start and soft stop. This 1MHz all-ceramic capacitor design uses the MAX8538 dual synchronous buck controller to provide soft-start and soft-stop functions for the +5V and -5V rails at the same time. One controller output is configured as a synchronous buck converter to provide +5V @ 3A. The second buck controller is configured as a flyback inverter with a 1:1 transformer to provide -5V @ 3A. The negative output voltage is controlled by a MAX4400 op-amp with an inverting gain of one-half. The op-amp is biased from the MAX8538's on-board 5V VL regulator. With the gain of -1/2 the op-amp output is within its range and within the common-mode range of the error amp. The control gain of a buck converter is Vout / Vin = duty cycle. The control gain of a 1:1 flyback inverter is Vout / Vin = duty cycle / (1 - duty cycle), which increases with duty cycle. For a 1:1 flyback inverter, duty cycle = Vout / (Vin + Vout) = 5V / (10.8V + 5V) = 0.317. Then the extra control gain is 1 / (1 - 0.317) = 1.464. This extra gain is factored into the design equations for Type 3 compensation by reducing the designed cross-over frequency a factor of 1.464. The op-amp inverting gain is compensated by adjusting R23 without affecting the rest of the compensation network. A MAX837 voltage monitor provides under-voltage lockout. The MAX8538 is turned on when Vin reaches 9.2V. R31 introduces 75mV of positive feedback (hysteresis) for noise-free switching to allow a clean soft-start and soft-stop with a low Vin slew-rate. Note that since the flyback inverter has discontinuous transformer current, getting the -5V soft-stop to be symmetric with the +5V rail will depend on the load on the -5V rail. Also the -5V side's discontinuous transformer current requires larger output filter capacitance than the +5V side. For Larger Image Figure 1. Table 1. Vin Iin V1 I1 V2 I2 Efficiency 12.00 0.044 5.083 0 -5.043 0   10.77 3.41 5.078 3 -5.038 3 0.826 12.00 3.06 5.080 3 -5.040 3 0.827 13.23 2.79 5.080 3 -5.040 3 0.823     20mVpp ripple in 20MHz BW   25mVpp ripple in 20MHz BW     Table 2. Bill of Materials 10.8V to 13.2V Input -5V @ 3A, 5V @ 3A Output 9/30/03 Designation QTY Description C2,C4 3 10uF 16V X5R ceramic capacitor (1210) Taiyo Yuden EMK325BJ106MN C5,C6,C7,C10, C41 5 0.1uF ceramic capacitor (0603) C12,C30 7 100uF 6V Y5U ceramic capacitor (1210) Taiyo Yuden JMK325E107MM C13 1 4.7uF 6V X5R ceramic capacitor (0805) Taiyo Yuden JMK212BJ475MG C14 1 1uF 25V X5R ceramic capacitor (1206) Taiyo Yuden TMK316BJ105ML C15 1 1uF 6V X5R ceramic capacitor (0603)Taiyo Yuden JMK107BJ105MA C17 1 3.3nF ceramic capacitor (0603) C18,C20 2 5pF ceramic capacitor (0603) C19 1 560pF ceramic capacitor (0603) C21 1 1.5nF ceramic capacitor (0603) C22 1 220pF ceramic capacitor (0603) C23,C24 2 33nF ceramic capacitor (0603) C40 1 680pF ceramic capacitor (0603) C42 1 1.2nF ceramic capacitor (0603) C43 1 470pF ceramic capacitor (0603) D1,D2 2 100mA 30V Schottky diode (SOD-323) Central Semi CMDSH-3 D3 1 5A 40V Schottky diode Central Semi CMSH5-40 L1 1 2.2uH 10A Power Inductor Delta THA104-2R2 L2 1 2.2uH 14.4A Dual Power Inductor Pulse P0395 N1,N5,N9 3 30V 13mΩ N-ch MOSFET (SO-8) International Rectifier IRF7821 N3,N7 2 30V 30mΩ N-ch MOSFET (SO-8) Fairchild FDS6612A R2 1 511 Ω 1% resistor (0603) R3 1 681 Ω 1% resistor (0603) R7 1 2.2 Ω 5% resistor (0603) R8 1 4.7 Ω 5% resistor (0603) R9,R24 2 20.0k Ω 1% resistor (0603) R10,R15 2 53.6k Ω 1% resistor (0603) R11 1 51k Ω 5% resistor (0603) R12 1 1.5k Ω 5% resistor (0603) R13 1 4.3k Ω 5% resistor (0603) R14 1 43k Ω 5% resistor (0603) R16,R25 2 10.0k Ω 1% resistor (0603) R22 1 5.1Ω 0.125W 5% resistor (1206) R23 1 24.9k Ω 1% resistor (0603) R26 1 3.3 Ω 0.25W 5% resistor (1210) R27 1 18 Ω 5% resistor (0603) R28 1 33 Ω 5% resistor (0603) R29 1 10.0k Ω 1% resistor (0603) R30 1 1.50k Ω 1% resistor (0603) R31 1 100k Ω 1% resistor (0603) U1 1 MAX8538EEI (28-QSOP) U2 1 MAX4400AXK (5-SC70) U3 1 MAX837EUS (4-SOT143) フィードバックをお寄せください。 内容に満足されましたか、あるいは満足されていませんか？もっと良いページにできると思いますか？あるいは、単なるコメントでも結構です。フィードバックをお待ちしています。—マキシムはお客様からいただく訂正、提案を元に改善していきます。 このページを評価し、フィードバックを送信する。 自動アップデート お客様が関心のある分野でアプリケーションノートが新規に掲載された際に自動通知Eメールの受信を希望する場合は、EE-Mail™にご登録ください。 その他の情報   APP 3046: Mar 26, 2004 MAX4400 シングル/デュアル/クワッド、低コスト、単一電源、レイルトゥレイルオペアンプ、シャットダウン付 フルデータシート (PDF, 248kB) 無料 サンプル MAX837 4ピン、マイクロパワー、電圧モニタ フルデータシート (PDF, 144kB) 無料 サンプル MAX8538 PoL型、トラッキング、およびDDRメモリ電源用、デュアル同期降圧型コントローラ フルデータシート (PDF, 328kB) 無料 サンプル   ダウンロード、PDFフォーマット (60kB)  AN3046, AN 3046, APP3046, Appnote3046, Appnote 3046