Step-down/inverting 3A, ±5V power supply with all-ceramic capacitor design

The 1MHz, all-ceramic capacitor design provided here uses the MAX8538 dual-synchronous buck controller to provide +5V and -5V power supplies, and has soft-start and soft-shutdown functions. One controller is configured for synchronous buck conversion, providing +5V @ 3A output, and the second buck controller is configured for flyback conversion, providing -5V @ 3A output through a 1:1 transformer.

A MAX4400 op amp with a gain of -1/2 controls the negative voltage output. The MAX8538's internal 5V VL linear regulator provides bias for the op amp . The output of an op amp with a gain of -1/2 is limited to this range and is within the common-mode limits of the error amplifier.

Buck converter control gain is: V _OUT /V _IN = duty cycle. The control gain of the 1:1 flyback converter is: V _OUT /V _IN = duty cycle/(1 - duty cycle), which increases as the duty cycle increases. For a 1:1 flyback converter, duty cycle = V _OUT /(V _IN + V _OUT ) = 5V/(10.8V + 5V) = 0.317. Thus, additional control gain = 1/(1 - 0.317) = 1.464. The additional control gain is a factor that affects the design of the Category 3 compensation network, reducing the unity gain frequency by a factor of 1.464. The inverting gain of the operational amplifier is compensated by adjusting R23, which will not affect other circuits of the compensation network.

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figure 1.