LM2599 datasheet(21/31 Pages) NSC

21 / 31 page

Application Information (Continued)

A graph shown in

Figure 15 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 up-

per 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 out-

put voltage. But if the selected capacitor’s ESR is extremely

low, there is a possibility of an unstable feedback loop, re-

sulting in an oscillation at the output. Using the capacitors

listed in the tables, or similar types, will provide design solu-

tions 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 16). 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 of-

ten can be satisfied with only three or four different capacitor

values and several different voltage ratings. See the quick

design component selection tables in

Figure 2 and 3 for typi-

cal capacitor values, voltage ratings, and manufacturers ca-

pacitor types.

Electrolytic capacitors are not recommended for tempera-

tures below −25˚C. The ESR rises dramatically at cold tem-

peratures and typically rises 3X @ −25˚C and as much as

10X at −40˚C. See curve shown in

Figure 17.

Solid tantalum capacitors have a much better ESR spec for

cold temperatures and are recommended for temperatures

below −25˚C.

CATCH DIODE

Buck regulators require a diode to provide a return path for

the inductor current when the switch turns off. This must be

a fast diode and must be located close to the LM2599 using

short leads and short printed circuit traces.

Because of their very fast switching speed and low forward

voltage drop, Schottky diodes provide the best performance,

especially in low output voltage applications (5V and lower).

Ultra-fast recovery, or High-Efficiency rectifiers are also a

good choice, but some types with an abrupt turnoff charac-

teristic may cause instability or EMI problems. Ultra-fast re-

covery diodes typically have reverse recovery times of 50 ns

or less. Rectifiers such as the IN5400 series are much too

slow and should not be used.

INDUCTOR SELECTION

All switching regulators have two basic modes of operation;

continuous and discontinuous. The difference between the

two types relates to the inductor current, whether it is flowing

continuously, or if it drops to zero for a period of time in the

normal switching cycle. Each mode has distinctively different

operating characteristics, which can affect the regulators

performance and requirements. Most switcher designs will

operate in the discontinuous mode when the load current is

low.

The LM2599 (or any of the Simple Switcher family) can be

used for both continuous or discontinuous modes of opera-

tion.

In many cases the preferred mode of operation is the con-

tinuous mode. It offers greater output power, lower peak

switch, inductor and diode currents, and can have lower out-

put ripple voltage. But it does require larger inductor values

to keep the inductor current flowing continuously, especially

at low output load currents and/or high input voltages.

To simplify the inductor selection process, an inductor selec-

tion guide (nomograph) was designed (see

Figure 4 through

7). This guide assumes that the regulator is operating in the

continuous mode, and selects an inductor that will allow a

peak-to-peak inductor ripple current to be a certain percent-

age of the maximum design load current. This peak-to-peak

inductor ripple current percentage is not fixed, but is allowed

to change as different design load currents are selected.

(See

Figure 18).

DS012582-35

FIGURE 17. Capacitor ESR Change vs Temperature

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