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Pushing control bandwidth for less output capacitors of Sigma Voltage Regulator

Year: 2010
Fig. 1. Control Bandwidth impact to output capacitors of Sigma VR.
Pushing the control bandwidth of Sigma VR can reduce the needed output bulk capacitors. Different types of output bulk capacitors have different impact to the control bandwidth as shown in Fig 1.

Using SP output capacitor, other than very small Lout value, pushing the bandwidth to be high is not helpful to reduce the output capacitor number due to the relatively large ESR during certain concerned frequency bandwidth range. But for the ceramic capacitor, pushing the bandwidth is able to continually reduce the output capacitor number due to the small ESR and the Lout value is not so critical for output capacitors.

The cost reduction and footprint reduction can be achieved by changing the output bulk capacitor from SP to ceramic capacitor shown in Fig 2. Below 100 kHz control bandwidth, SP bulk capacitors have the cost benefit compared with the ceramic capacitors. However, beyond 100 kHz, ceramic capacitor begins to show the benefit both from the cost and footprint. Thus, there is a hint that if we want to push the control bandwidth, changing output bulk capacitors to ceramic capacitors is a most cost-effective solution. What is more, due to the low ESR effect, further pushing the control can further reduce the output ceramic bulk capacitor number.

By pushing the control bandwidth from 100 kHz to 200 kHz could continually reduce the output ceramic capacitor number from 12 to just 3 which cannot be achieved by using SP bulk capacitors. Greater cost and footprint saving can be achieved by further pushing the control bandwidth.

Fig. 2. Cost and Footprint reduction by using ceramic capacitors.

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