LM2595 datasheet(21/43 Pages) TI1

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LM2595

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SNVS122C – MAY 1999 – REVISED MAY 2016

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The output ripple voltage is a function of the inductor sawtooth ripple current and the ESR of the output

capacitor. A typical output ripple voltage can range from approximately 0.5% to 3% of the output voltage. To

obtain low ripple voltage, the ESR of the output capacitor must be low; however, caution must be exercised when

using extremely low ESR capacitors because they can affect the loop stability, resulting in oscillation problems. If

very low output ripple voltage is required (less than 20 mV), TI recommends a post ripple filter (see Figure 32).

The inductance required is typically between 1 μH and 5 μH, with low DC resistance, to maintain good load

regulation. A low ESR output filter capacitor is also required to assure good dynamic load response and ripple

reduction. The ESR of this capacitor may be as low as desired, because it is out of the regulator feedback loop.

Figure 22 shows a typical output ripple voltage, with and without a post ripple filter.

When observing output ripple with a scope, it is essential that a short, low inductance scope probe ground

connection be used. Most scope probe manufacturers provide a special probe terminator which is soldered onto

the regulator board, preferably at the output capacitor. This provides a very short scope ground, thus eliminating

the problems associated with the 3 inch ground lead normally provided with the probe, and provides a much

cleaner and more accurate picture of the ripple voltage waveform.

The voltage spikes are caused by the fast switching action of the output switch and the diode, the parasitic

inductance of the output filter capacitor, and its associated wiring. To minimize these voltage spikes, the output

capacitor must be designed for switching regulator applications, and the lead lengths must be kept very short.

Wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all contribute

to the amplitude of these spikes.

When a switching regulator 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 and output voltage,

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

decreases, the entire sawtooth current waveform also rises and falls. The average value (or the center) of this

current waveform is equal to the DC load current.

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

the switcher will smoothly change from a continuous to a discontinuous mode of operation. Most switcher

designs (regardless how large the inductor value is) will be forced to run discontinuous if the output is lightly

loaded. This is a perfectly acceptable mode of operation.

Figure 31. Peak-to-Peak Inductor

Ripple Current vs Load Current

In a switching regulator design, knowing the value of the peak-to-peak inductor ripple current (ΔIIND) can be

useful for determining a number of other circuit parameters. Parameters such as, peak inductor or peak switch

current, minimum load current before the circuit becomes discontinuous, output ripple voltage and output

capacitor ESR can all be calculated from the peak-to-peak ΔIIND. When the inductor nomographs shown in

Figure 27 through Figure 30 are used to select an inductor value, the peak-to-peak inductor ripple current can

immediately be determined. Figure 31 shows the range of (ΔIIND) that can be expected for different load currents.

Figure 31 also shows how the peak-to-peak inductor ripple current (ΔIIND) changes from the lower border to the

upper border (for a given load current) within an inductance region. The upper border represents a higher input

voltage, while the lower border represents a lower input voltage (see Inductor Selection).

These curves are only correct for continuous mode operation, and only if the inductor selection guides are used

to select the inductor value

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