Datasheet

LM2597, LM2597HV

SNVS119C –MARCH 1998–REVISED APRIL 2013

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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.

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 36). 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 1 and Table 2 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 37.

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 LM2594 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 1N4001 series are much too slow and should not be used.

Product Folder Links: LM2597 LM2597HV