Reference Circuit 3046

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.

Figure 1.

For Larger Image

Figure 1.

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.

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)

 

 
Status:
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MAX8538
Dual-Synchronous Buck Controllers for Point-of-Load, Tracking, and DDR Memory Power Supplies

  • MAX8537/MAX8539: Complete DDR Supplies
  • MAX8538: Dual Nontracking Controller
  • Out-of-Phase (MAX8537/MAX8538) or In-Phase (MAX8539) Operation

MAX4400
Single/Dual/Quad, Low-Cost, Single-Supply, Rail-to-Rail Op Amps with Shutdown

  • Single +2.5V to +5.5V Supply Voltage Range
  • 320µA Quiescent Current per Amplifier
  • 1µA (max) Shutdown Mode (MAX4401)

MAX837
4-Pin Micropower Voltage Monitors

  • ±1.25% Precision Voltage Threshold
  • SOT143 Package
  • Low Cost