LM2576 datasheet(16/46 Pages) TI

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more electromagnetic interference (EMI). This EMI can cause problems in sensitive circuits, or can give incorrect

scope readings because of induced voltages in the scope probe.

The inductors listed in the selection chart include ferrite pot core construction for AIE, powdered iron toroid for

Pulse Engineering, and ferrite bobbin core for Renco.

An inductor must not operate beyond its maximum-rated current because it can 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), causing the switch current to rise very rapidly. Different inductor types have different

saturation characteristics, and this must be considered when selecting an inductor.

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

8.1.3 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 reaches zero, and

the switcher changes 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) is forced to run discontinuous if the load

current is light enough.

8.1.4 Output Capacitor

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

placed near the LM2576 using short PCB traces. Standard aluminum electrolytics are usually adequate, but TI

recommends low ESR types for low output ripple voltage and good stability. The ESR of a capacitor depends on

many factors, including: the value, the voltage rating, physical size, and the type of construction. In general, low

value or low voltage (less than 12 V) 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 (ΔIIND). See Section 8.1.3.

The lower capacitor values (220 μF to 1000 μF) allows typically 50 mV to 150 mV of output ripple voltage, while

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

Output Ripple Voltage = (ΔIIND) (ESR of COUT)

(5)

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

grade capacitor can be used. Such capacitors are often called high-frequency, low-inductance, or low-ESR.

These reduces the output ripple to 10 mV or 20 mV. However, when operating in the continuous mode, reducing

the ESR below 0.03 Ω can cause instability in the regulator.

Tantalum capacitors can have a very low ESR, and must 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 ripple current rating of the capacitor at 52 kHz must be at least 50% higher than the peak-to-peak inductor

ripple current.

8.1.5 Catch Diode

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

must be placed close to the LM2576 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 5 V). Fast-recovery, high-efficiency, or ultra-fast

recovery diodes are also suitable, but some types with an abrupt turnoff characteristic can cause instability

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

LM2576, LM2576HV

SNVS107G – JUNE 1999 – REVISED MARCH 2023

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