LM2597 datasheet(23/34 Pages) NSC

23 / 34 page

Application Information (Continued)

current rating rather than its capacitance or voltage ratings,

although the capacitance value and voltage rating are di-

rectly related to the RMS current rating.

The RMS current rating of a capacitor could be viewed as a

capacitor’s power rating. The RMS current flowing through

the capacitors internal ESR produces power which causes

the internal temperature of the capacitor to rise. The RMS

current rating of a capacitor is determined by the amount of

current required to raise the internal temperature approxi-

mately 10˚C above an ambient temperature of 105˚C. The

ability of the capacitor to dissipate this heat to the surround-

ing air will determine the amount of current the capacitor can

safely sustain. Capacitors that are physically large and have

a large surface area will typically have higher RMS current

ratings. For a given capacitor value, a higher voltage elec-

trolytic capacitor will be physically larger than a lower voltage

capacitor, and thus be able to dissipate more heat to the

surrounding air, and therefore will have a higher RMS cur-

rent rating.

The consequences of operating an electrolytic capacitor

above the RMS current rating is a shortened operating life.

The higher temperature speeds up the evaporation of the

capacitor’s electrolyte, resulting in eventual failure.

Selecting an input capacitor requires consulting the manu-

facturers data sheet for maximum allowable RMS ripple

current. For a maximum ambient temperature of 40˚C, a

general guideline would be to select a capacitor with a ripple

current rating of approximately 50% of the DC load current.

For ambient temperatures up to 70˚C, a current rating of

75% of the DC load current would be a good choice for a

conservative design. The capacitor voltage rating must be at

least 1.25 times greater than the maximum input voltage,

and often a much higher voltage capacitor is needed to

satisfy the RMS current requirements.

A graph shown in

Figure 16 shows the relationship between

an electrolytic capacitor value, its voltage rating, and the

RMS current it is rated for. These curves were obtained from

the Nichicon “PL” series of low ESR, high reliability electro-

lytic capacitors designed for switching regulator applications.

Other capacitor manufacturers offer similar types of capaci-

tors, but always check the capacitor data sheet.

“Standard” electrolytic capacitors typically have much higher

ESR numbers, lower RMS current ratings and typically have

a shorter operating lifetime.

Because of their small size and excellent performance, sur-

face mount solid tantalum capacitors are often used for input

bypassing, but several precautions must be observed. A

small percentage of solid tantalum capacitors can short if the

inrush current rating is exceeded. This can happen at turn on

when the input voltage is suddenly applied, and of course,

higher input voltages produce higher inrush currents. Sev-

eral capacitor manufacturers do a 100% surge current test-

ing on their products to minimize this potential problem. If

high turn on currents are expected, it may be necessary to

limit this current by adding either some resistance or induc-

tance before the tantalum capacitor, or select a higher volt-

age capacitor. As with aluminum electrolytic capacitors, the

RMS ripple current rating must be sized to the load current.

OUTPUT CAPACITOR

OUT — An output capacitor is required to filter the output

and provide regulator loop stability. Low impedance or low

ESR Electrolytic or solid tantalum capacitors designed for

switching regulator applications must be used. When select-

ing an output capacitor, the important capacitor parameters

are; the 100 kHz Equivalent Series Resistance (ESR), the

RMS ripple current rating, voltage rating, and capacitance

value. For the output capacitor, the ESR value is the most

important parameter.

The output capacitor requires an ESR value that has an

upper and lower limit. For low output ripple voltage, a low

ESR value is needed. This value is determined by the maxi-

mum allowable output ripple voltage, typically 1% to 2% of

the output voltage. But if the selected capacitor’s ESR is

extremely low, there is a possibility of an unstable feedback

loop, resulting in an oscillation at the output. Using the

capacitors listed in the tables, or similar types, will provide

design solutions under all conditions.

If very low output ripple voltage (less than 15 mV) is re-

quired, refer to the section on Output Voltage Ripple and

Transients for a post ripple filter.

An aluminum electrolytic capacitor’s ESR value is related to

the capacitance value and its voltage rating. In most cases,

Higher voltage electrolytic capacitors have lower ESR values

(see

Figure 17). Often, capacitors with much higher voltage

ratings may be needed to provide the low ESR values re-

quired for low output ripple voltage.

The output capacitor for many different switcher designs

often can be satisfied with only three or four different capaci-

tor values and several different voltage ratings. See the

DS012440-28

FIGURE 16. RMS Current Ratings for Low

ESR Electrolytic Capacitors (Typical)

DS012440-29

FIGURE 17. Capacitor ESR vs Capacitor Voltage Rating

(Typical Low ESR Electrolytic Capacitor)

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