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Non-Isolated DC/DC Forum
O/p cap in DC-DC or regulators
1. I read tow purposes of o/p cap :- gnd ripple noises & improving transient responses.
2. In Dc-DC, when we write o/p cap is used in controlling feedback loop, what does that mean. What loop it is stablizing.
3. o/p caps should have lowest ESR, right. But I saw an examole where certain ESR values of cap are defined. E.g for some freq range to get stable o/p, ESR value should be 20mohm & other freq range ESR value should be 30-50milliohm.
What if I use lower ESR values as specified.
The questions you ask are general in nature and teh answers will be somewhat dependent on your specific topology.
1. Yes, the primary need for the output capacitor is to filter ripple on the output and supply energy to the load during transient conditions (or in the case of a boost converter to supply current when the inductor si charging from the input).
2. Modern dc/dc converters use a feed back control loop to regulate the output voltage to a fixed voltage. This control loop must be stabilized to keep the output from oscillating. The output filter components are part of the closed control loop.
3. For lowest ripple, low ESR is preferred. To reduce component count, some specific control implemetations use the ESR zero of the output capacitor (1/2*pi*Cout*Resr) to help stabilize the loop. Devices with external compensation may not have any restriction on low ESR. Devices with internal compensation may require both capacitance and ESR in a specific range to ensure stablity. You do not specify a device, but in general you should stay within the datasheet recommendations.
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