Synchronized Ripple Currents

A ring counter generates three synchronization signals at 600kHz, 33% duty cycle, phased 120° apart. The sync input will operate over a wide range of duty cycles, so no further pulse conditioning is needed. At full load, each device's input ripple current is a 4A trapezoidal wave at 600kHz, as shown in Figure 45. Summing these waveforms gives the effective input ripple for the complete system. The resultant waveform, shown at the bottom of Figure 45, remains at 4A but its frequency has increased to 1.8MHz. The higher frequency eases the requirements on the value of input filter without the 3x increase in ripple current rating that would normally occur. Although only a single input capacitor is required, practical layout restrictions usually dictate an individual capacitor at each device. Figure 46 shows the output ripple current waveforms. The resultant 1.8MHz triangular waveform has a maximum amplitude of 350mA at an input voltage of 10V. This is significantly lower than would be expected for a 12A output. Interestingly, at inputs of 7.6V and 15V, the theoretical summed output ripple current cancels completely. To reduce board space and ripple voltage, C1 and C3 are ceramic capacitors. Loop compensation capacitor C4 must be adjusted when using ceramic output capacitors, due to the lack of effective series resistance (ESR).

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