LM2598 datasheet(21/47 Pages) TI1

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LM2598

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SNVS125D – MARCH 1998 – REVISED MAY 2016

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Application Information (continued)

Figure 31 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 electrolytic

capacitors designed for switching regulator applications. Other capacitor manufacturers offer similar types of

capacitors, 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, surface 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 turnon when the input voltage is suddenly

applied, and of course, higher input voltages produce higher inrush currents. Several capacitor manufacturers do

a 100% surge current testing on their products to minimize this potential problem. If high turnon currents are

expected, it may be necessary to limit this current by adding either some resistance or inductance before the

tantalum capacitor, or select a higher voltage capacitor. As with aluminum electrolytic capacitors, the RMS ripple

current rating must be sized to the load current.

9.1.5 Output Capacitor (COUT)

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

selecting 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 required. This value is determined by the maximum 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, provides design solutions under all conditions.

If very low output ripple voltage (less than 15 mV) is required, see 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 32). Often, capacitors with much

higher voltage ratings may be required to provide the low ESR values required for low output ripple voltage.

The output capacitor for many different switcher designs often can be satisfied with only three or four different

capacitor values and several different voltage ratings. See Figure 38 and Table 1 for typical capacitor values,

voltage ratings, and manufacturers capacitor types.

Electrolytic capacitors are not recommended for temperatures below –25°C. The ESR rises dramatically at cold

temperatures and typically rises 3X at –25°C and as much as 10X at –40°C. See curve shown in Figure 33.

Solid tantalum capacitors have a much better ESR specifications for cold temperatures and are recommended

for temperatures below –25°C.

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