LM2599 datasheet(23/31 Pages) NSC

23 / 31 page

Application Information (Continued)

mended (See

Figure 1). 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 de-

sired, because it is out of the regulator feedback loop. The

photo shown in

Figure 19 shows a typical output ripple volt-

age, 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,

preferable 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, the diode, and the parasitic inductance of

the output filter capacitor, and its associated wiring. To mini-

mize these voltage spikes, the output capacitor should be

designed for switching regulator applications, and the lead

lengths must be kept very short. Wiring inductance, stray ca-

pacitance, 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 triangu-

lar 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 re-

mains 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 discon-

tinuous mode of operation. Most switcher designs (irregard-

less how large the inductor value is) will be forced to run dis-

continuous if the output is lightly loaded. This is a perfectly

acceptable mode of operation.

In a switching regulator design, knowing the value of the

peak-to-peak inductor ripple current (

∆I

IND) can be useful for

determining a number of other circuit parameters. Param-

eters such as, peak inductor or peak switch current, mini-

mum load current before the circuit becomes discontinuous,

output ripple voltage and output capacitor ESR can all be

calculated from the peak-to-peak

∆I

IND. When the inductor

nomographs shown in

Figure 4 through 7 are used to select

an inductor value, the peak-to-peak inductor ripple current

can immediately be determined. The curve shown in

Figure

20 shows the range of (

∆I

IND) that can be expected for differ-

ent load currents. The curve also shows how the

peak-to-peak inductor ripple current (

∆I

IND) changes as you

go from the lower border to the upper border (for a given load

current) within an inductance region. The upper border rep-

resents a higher input voltage, while the lower border repre-

sents a lower input voltage (see Inductor Selection Guides).

These curves are only correct for continuous mode opera-

tion, and only if the inductor selection guides are used to se-

lect the inductor value

Consider the following example:

OUT = 5V, maximum load current of 2.5A

IN = 12V, nominal, varying between 10V and 16V.

The selection guide in

Figure 5 shows that the vertical line

for a 2.5A load current, and the horizontal line for the 12V in-

put voltage intersect approximately midway between the up-

per and lower borders of the 33 µH inductance region. A 33

µH inductor will allow a peak-to-peak inductor current (

∆I

IND)

to flow that will be a percentage of the maximum load cur-

rent. Referring to

Figure 20, follow the 2.5A line approxi-

mately midway into the inductance region, and read the

peak-to-peak inductor ripple current (

∆I

IND) on the left hand

axis (approximately 620 mA p-p).

As the input voltage increases to 16V, it approaches the up-

per border of the inductance region, and the inductor ripple

current increases. Referring to the curve in

Figure 20,itcan

be seen that for a load current of 2.5A, the peak-to-peak in-

ductor ripple current (

∆I

IND) is 620 mA with 12V in, and can

range from 740 mA at the upper border (16V in) to 500 mA at

the lower border (10V in).

Once the

∆I

IND value is known, the following formulas can be

used to calculate additional information about the switching

regulator circuit.

Peak Inductor or peak switch current

Minimum load current before the circuit becomes dis-

continuous

Output Ripple Voltage = (

∆I

IND)x(ESR of COUT)

= 0.62Ax0.1

Ω=62 mV p-p

OPEN CORE INDUCTORS

Another possible source of increased output ripple voltage or

unstable operation is from an open core inductor. Ferrite

bobbin or stick inductors have magnetic lines of flux flowing

through the air from one end of the bobbin to the other end.

DS012582-49

FIGURE 20. Peak-to-Peak Inductor

Ripple Current vs Load Current

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