Datasheet

LM2599

SNVS123C –APRIL 1998–REVISED APRIL 2013

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Figure 33. Capacitor ESR vs Capacitor Voltage Rating (Typical Low ESR Electrolytic Capacitor)

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 manufacturers 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 32 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 turn on 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 turn on 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.

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 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 needed. 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, will provide design solutions under all conditions.

If very low output ripple voltage (less than 15 mV) is required, refer to the section on OUTPUT VOLTAGE

RIPPLE AND TRANSIENTS for a post ripple filter.

Product Folder Links: LM2599