Greetings to Dr. FAE,
I notice many of your competitors do not use feedback loops in their reference designs. You guys have some parts with no feedback loop (like the tiny 0.6A 6MHz FAN5361 regulator http://www.fairchildsemi.com/pf/FA/FAN5361.html) while some parts, like your new 4A integrated regulator FAN21SV04 (http://www.fairchildsemi.com/pf/FA/FAN21SV04.html) do have an external feedback loop. I wonder: why is this? If I have a choice, I don’t want to mess around with external parts and complicated circuit analysis. It would make my life easier if you’d quit making parts with feedback loops.  – – – Baffled in Buffalo

Dear Mister Baffled,

If we’re talking about making people’s lives easier, you could connect our parts to well-behaved loads with well-behaved sources and we could sell you expensive resistors in fancy packages and make a lot of money.

However, most parts that provide a regulated output have a feedback loop for control and enhanced stability. Sometimes this feedback loop is buried inside the part…in that case we design a control loop we think will work well for a variety of circuits a customer might hook it to. Even a simple device like a Low-Dropout (LDO) regulator has a feedback loop from the output to control the conduction of the pass transistor. This is why, in rare situations, when a load is ill-behaved or a board layout is poor or bypass capacitors are improperly selected-the output will oscillate. In a feedback loop, we’re always trying to balance the aggressiveness of the response (the speed that the regulator will respond to load changes) with stability over temperature, component variation, worst case circuit and device influences. This brings up subjects like Bode plots and phase/gain margin that are uncomfortable for some people.

So, when you look at designing with a part like the FAN21SV04 and see the external feedback loop components, we’re trying to do you a service. You can adjust the feedback compensation to meet the transient requirements of your design.  So I hope you agree – including a feedback loop will make your life easier.

Now if you’ll excuse me, I have to visit my therapist.

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