All-Ceramic Capacitor Step-Down/Inverter Supplies ±5V at 3A

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.

Figure 1.
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
MAX4400 Single/Dual/Quad, Low-Cost, Single-Supply, Rail-to-Rail Op Amps with Shutdown Samples  
MAX837 4-Pin Micropower Voltage Monitors Samples  
MAX8538 Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power Supplies Samples  

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© Mar 26, 2004, Maxim Integrated Products, Inc.
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APP 3046: Mar 26, 2004
REFERENCE SCHEMATIC 3046, AN3046, AN 3046, APP3046, Appnote3046, Appnote 3046