Datasheet

LM2599

www.ti.com

SNVS123C –APRIL 1998–REVISED APRIL 2013

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 33). Often, capacitors with much

higher voltage ratings may be needed 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 the quick design component selection tables in

Table 2 and Table 4 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 @ −25°C and as much as 10X at −40°C. See curve shown in Figure 34.

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 characteristic may cause instability or

EMI problems. Ultra-fast recovery 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.

Figure 34. Capacitor ESR Change vs Temperature

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

operation.

In many cases the preferred mode of operation is the continuous mode. It offers greater output power, lower

peak switch, inductor and diode currents, and can have lower output 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 selection guide (nomograph) was designed (see Figure 25

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 percentage 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 35).

Product Folder Links: LM2599