All-Ceramic Capacitor Step-Down/Inverter Supplies ±5V at 3A
This 1MHz all-ceramic capacitor design uses the 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.
| 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 | 0.823 |
| 13.21 | 2.79 | 5.080 | 3 | -5.040 | 3 | 0.823 |
| 20mVP-P ripple in 20MHz BW | 25mVP-P ripple in 20MHz BW |
| 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 | 4.7kΩ 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 | 3.3kΩ .025W 5% resistor (1210) |
| R27 | 1 | 18Ω 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) |
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