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 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.
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)
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