Abstract: A MAX8538 dual synchronous buck controller supplies both +5V and -5V outputs at 3A. One controller operates as a synchronous rectified 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 at 3A. The second buck controller is configured as a flyback inverter with a 1:1 transformer to provide -5V at 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     20mVP-P ripple in 20MHz BW   25mVP-P ripple in 20MHz BW     Table 2. Bill of Materials 10.8V to 13.2V Input; -5V at 3A, 5V at 3A Output Designation QTY Description C2, C4 3 10µF 16V X5R ceramic capacitor (1210) Taiyo Yuden EMK325BJ106MN C5, C6, C7, C10, C41 5 0.1µF ceramic capacitor (0603) C12, C30 7 100µF 6V Y5U ceramic capacitor (1210) Taiyo Yuden JMK325E107MM C13 1 4.7µF 6V X5R ceramic capacitor (0805) Taiyo Yuden JMK212BJ475MG C14 1 1µF 25V X5R ceramic capacitor (1206) Taiyo Yuden TMK316BJ105ML C15 1 1µF 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 2.2µH 10A Power Inductor Delta THA104-2R2 L2 1 2.2µH 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 511Ω 1% resistor (0603) R3 1 681Ω 1% resistor (0603) R7 1 2.2Ω 5% resistor (0603) R8 1 4.7Ω 5% resistor (0603) R9, R24 2 20.0kΩ 1% resistor (0603) R10, R15 2 53.6kΩ 1% resistor (0603) R11 1 51kΩ 5% resistor (0603) R12 1 1.5kΩ 5% resistor (0603) R13 1 4.3kΩ 5% resistor (0603) R14 1 43kΩ 5% resistor (0603) R16, R25 2 10.0kΩ 1% resistor (0603) R22 1 5.1Ω 0.125W 5% resistor (1206) R23 1 24.9kΩ 1% resistor (0603) R26 1 3.3Ω 0.25W 5% resistor (1210) R27 1 18Ω 5% resistor (0603) R28 1 33Ω 5% resistor (0603) R29 1 10.0kΩ 1% resistor (0603) R30 1 1.50kΩ 1% resistor (0603) R31 1 100kΩ 1% resistor (0603) U1 1 MAX8538EEI (28 QSOP) U2 1 MAX4400AXK (5 SC70) U3 1 MAX837EUS (4 SOT143) Related Parts   APP 3046: Mar 26, 2004 MAX4400 Single/Dual/Quad, Low-Cost, Single-Supply, Rail-to-Rail Op Amps with Shutdown Full Data Sheet (PDF, 384kB) Free Samples MAX837 4-Pin Micropower Voltage Monitors Full Data Sheet (PDF, 108kB) Free Samples MAX8538 Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power Supplies Full Data Sheet (PDF, 328kB) Free Samples Automatic Updates Would you like to be automatically notified when new application notes are published in your areas of interest? Sign up for EE-Mail™. We Want Your Feedback!   Download, PDF Format (35kB)  AN3046, AN 3046, APP3046, Appnote3046, Appnote 3046