LM1575, LM2575-N, LM2575HV

SNVS106E – MAY 1999 – REVISED APRIL 2013

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An inductor should not be operated beyond its maximum rated current because it may saturate. When an

inductor begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive (the

DC resistance of the winding). This will cause the switch current to rise very rapidly. Different inductor types have

different saturation characteristics, and this should be kept in mind when selecting an inductor.

The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.

INDUCTOR RIPPLE CURRENT

When the switcher is operating in the continuous mode, the inductor current waveform ranges from a triangular

to a sawtooth type of waveform (depending on the input voltage). For a given input voltage and output voltage,

the peak-to-peak amplitude of this inductor current waveform remains constant. As the load current rises or falls,

the entire sawtooth current waveform also rises or falls. The average DC value of this waveform is equal to the

DC load current (in the buck regulator configuration).

If the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and

the switcher will change to a discontinuous mode of operation. This is a perfectly acceptable mode of operation.

Any buck switching regulator (no matter how large the inductor value is) will be forced to run discontinuous if the

load current is light enough.

OUTPUT CAPACITOR

An output capacitor is required to filter the output voltage and is needed for loop stability. The capacitor should

be located near the LM2575 using short pc board traces. Standard aluminum electrolytics are usually adequate,

but low ESR types are recommended for low output ripple voltage and good stability. The ESR of a capacitor

depends on many factors, some which are: the value, the voltage rating, physical size and the type of

construction. In general, low value or low voltage (less than 12V) electrolytic capacitors usually have higher ESR

numbers.

The amount of output ripple voltage is primarily a function of the ESR (Equivalent Series Resistance) of the

output capacitor and the amplitude of the inductor ripple current (

The lower capacitor values (220

μF–680 μF) will allow typically 50 mV to 150 mV of output ripple voltage, while

larger-value capacitors will reduce the ripple to approximately 20 mV to 50 mV.

Output Ripple Voltage = (

(9)

To further reduce the output ripple voltage, several standard electrolytic capacitors may be paralleled, or a

higher-grade capacitor may be used. Such capacitors are often called “high-frequency,” “low-inductance,” or

“low-ESR.” These will reduce the output ripple to 10 mV or 20 mV. However, when operating in the continuous

mode, reducing the ESR below 0.05

Ω can cause instability in the regulator.

Tantalum capacitors can have a very low ESR, and should be carefully evaluated if it is the only output capacitor.

Because of their good low temperature characteristics, a tantalum can be used in parallel with aluminum

electrolytics, with the tantalum making up 10% or 20% of the total capacitance.

The capacitor's ripple current rating at 52 kHz should be at least 50% higher than the peak-to-peak inductor

ripple current.

CATCH DIODE

Buck regulators require a diode to provide a return path for the inductor current when the switch is off. This diode

should be located close to the LM2575 using short leads and short printed circuit traces.

Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best efficiency,

especially in low output voltage switching regulators (less than 5V). Fast-Recovery, High-Efficiency, or Ultra-Fast

Recovery diodes are also suitable, but some types with an abrupt turn-off characteristic may cause instability and

EMI problems. A fast-recovery diode with soft recovery characteristics is a better choice. Standard 60 Hz diodes

(example: 1N4001 or 1N5400, and so on.) are also not suitable. See Table 1 for Schottky and “soft” fast-

recovery diode selection guide.

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