Summary:
The 1MHz, all-ceramic capacitor design offered here uses the MAX8538 dual-synchronous buck controller to provide +5V and -5V supplies with soft-start, soft-shutdown. The controller is configured for synchronous buck conversion with +5V @ 3A output, the second buck controller is configured for flyback conversion, and the -5V @ 3A output is provided by a 1:1 transformer.
The MAX4400, an op amp with a gain of -1/2, controls the negative voltage output. The MAX8538's internal 5V VL linear regulator provides bias to the op amp. The op amp output with a gain of -1/2 is limited to this range and is within the common mode limits of the error amplifier.
The Buck converter controls the gain to be: VOUT/VIN = duty cycle. The 1:1 flyback converter controls the gain to be: VOUT/VIN = duty cycle / (1 - duty cycle), which increases as the duty cycle increases. For a 1:1 flyback converter, duty cycle = VOUT / (VIN + VOUT) = 5V / (10.8V + 5V) = 0.317. Thus, the additional control gain = 1 / (1 - 0.317) = 1.464. The additional control gain is a factor affecting the design of the Class 3 compensation network, which reduces the unity gain frequency by 1.464 times. The operational amplifier amplifier has an inverting gain that is compensated by adjusting R23 and does not affect other circuits of the compensation network.
The MAX837 voltage sense provides undervoltage lockout. When the input voltage VIN is greater than 9.2V, the MAX8538 turns on. R31 introduces a positive feedback (hysteresis) of 75mV to avoid noise interference and provide a clean soft-start and soft-off process at a lower Vin slew rate. Note that since the flyback converter's transformer operates in a non-continuous current state, the symmetry of the -5V soft-off and +5V waveforms depends on the load of the -5V supply. In addition, because the current is not continuous on the -5V side, it is necessary to connect an output filter capacitor larger than the +5V output capacitor.
Provide buck / inverting 3A, ± 5V power supply circuit diagram
See the uploading documentation for details (click to download)
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