> Hi Jai,
>
> Switching supplies can be incredibly simple with the new devices that
> are out there and they can easily deliver the expected current with the
> suggested circuit from the Maxim data sheet.
>
> However, there are some fundamental rules that need to be followed.
> You're dealing with a high frequency, high power design and so circuit
> layout becomes an important issue. That means if you are having
> problems, the breadboard or vero board would be the first suspicious
> element to remove.
>
> *** So a proper layout is very important to how well this type of
> circuit will function. ***
>
> I haven't checked the recommended inductor on the data sheet but usually
> a number of inductor manufacturers and their part numbers are listed.
> If sounds like, from your response on the inductor issue that you aren't
> clear on saturation current of an inductor. I suspect you don't have a
> scope to look at waveforms either.
>
> So a bit of theory. When you apply a voltage across an inductor and
> measure the voltage at that instant in time, you will measure the
> applied voltage. If at that same instant in time you measure the
> current it will be 0. After an infinite amount of time the voltage will
> be zero and the current will be infinite. That's the theoretical
> inductor.
>
> In reality, the inductor has a certain amount of resistance and the
> power supply has limitations on current so one of two things will
> happen: the current increases to a point where the I^2R of the inductor
> wire is exceeded and it heats up and goes open circuit; the power
> supply goes into current limit and holds the current at Xi and the
> voltage drops to the I*R value across the inductor.
>
> So far pretty simple.
>
> What happens in-between is what is interesting. Simply put, if you were
> to watch the current flow on a scope you'd see an exponential increase
> in current but when the magnetic field created by this current saturates
> the core, there will be a dramatic almost avalanche like increase in
> current without any subsequent increase in energy storage. The coil
> heats up and could toast the driver transistor.
>
> So if you are using a 47uH inductor that saturates at 20ma then trying
> to run 500ma through the inductor will probably toast the regulator
> because the current will increase way beyond the specifications of the
> device before it has a chance to shut down the current; unless the
> inductor burns out first.
>
> When specifying an inductor, make sure that the current rating of the
> inductor is at least the value of the regulation device. For example:
> The Talema inductors listed as being suitable for National Semi Simple
> Switchers in the DigiKey catalog has and inductor 54uH at 1A and another
> one 53 uH at 3A. The amperage rating has far more to do with core
> saturation than wire size and heating since the 1A coil has a DCR of
> 0.20 Ohms while the 3A device has a DCR of 0.13 Ohms.
>
> Hope that helps.
>
> John Dammeyer
>
>
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>
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