LM26480 datasheet(25/30 Pages) NSC

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LDO CAPACITOR SELECTION

Input Capacitor

An input capacitor is required for stability. It is recommended

that a 1.0

μF capacitor be connected between the LDO input

pin and ground (this capacitance value may be increased

without limit). This capacitor must be located a distance of not

more than 1 cm from the input pin and returned to a clean

analog ground. Any good quality ceramic, tantalum, or film

capacitor may be used at the input.

Warning:

Important: Tantalum capacitors can suffer catastrophic failures

due to surge currents when connected to a low impedance

source of power (like a battery or a very large capacitor). If a

tantalum capacitor is used at the input, it must be guaranteed

by the manufacturer to have a surge current rating sufficient for

the application.

There are no requirements for the ESR on the input capacitor,

but tolerance and temperature coefficient must be considered

when selecting the capacitor to ensure the capacitance will re-

main approximately 1.0

μF over the entire operating tempera-

ture range.

Output Capacitor

The LDOs on the LM26480 are designed specifically to work

with very small ceramic output capacitors. A 1.0

μF ceramic

capacitor (temperature types Z5U, Y5V or X7R) with ESR be-

tween 5 m

Ω to 500 mΩ, are suitable in the application circuit.

It is also possible to use tantalum or film capacitors at the

device output COUT (or VOUT), but these are not as attrac-

tive for reasons of size and cost. The output capacitor must

meet the requirement for the minimum value of capacitance

and also have an ESR value that is within the range 5 m

Ω to

500 m

Ω for stability.

Capacitor Characteristics

The LDOs are designed to work with ceramic capacitors on

the output to take advantage of the benefits they offer. For

capacitance values in the range of 0.47

μF to 4.7 μF, ceramic

capacitors are the smallest, least expensive and have the

lowest ESR values, thus making them best for eliminating

high frequency noise. The ESR of a typical 1.0

μF ceramic

capacitor is in the range of 20 m

Ω to 40 mΩ, which easily

meets the ESR requirement for stability for the LDOs.

For both input and output capacitors, careful interpretation of

the capacitor specification is required to ensure correct device

operation. The capacitor value can change greatly, depend-

ing on the operating conditions and capacitor type.

In particular, the output capacitor selection should take ac-

count of all the capacitor parameters, to ensure that the

specification is met within the application. The capacitance

can vary with DC bias conditions as well as temperature and

frequency of operation. Capacitor values will also show some

decrease over time due to aging. The capacitor parameters

are also dependent on the particular case size, with smaller

sizes giving poorer performance figures in general. As an ex-

ample, the graph below shows a typical graph comparing

different capacitor case sizes in a capacitance vs. DC bias

plot.

30040416

As shown in the graph, increasing the DC bias condition can

result in the capacitance value that falls below the minimum

value given in the recommended capacitor specifications ta-

ble. Note that the graph shows the capacitance out of spec

for the 0402 case size capacitor at higher bias voltages. It is

therefore recommended that the capacitor manufacturers’

specifications for the nominal value capacitor are consulted

for all conditions, as some capacitor sizes (e.g. 0402) may not

be suitable in the actual application.

The ceramic capacitor’s capacitance can vary with tempera-

ture. The capacitor type X7R, which operates over a temper-

ature range of −55°C to +125°C, will only vary the capacitance

to within ±15%. The capacitor type X5R has a similar toler-

ance over a reduced temperature range of −55°C to +85°C.

Many large value ceramic capacitors, larger than 1

μF are

manufactured with Z5U or Y5V temperature characteristics.

Their capacitance can drop by more than 50% as the tem-

perature varies from 25°C to 85°C. Therefore X7R is recom-

mended over Z5U and Y5V in applications where the ambient

temperature will change significantly above or below 25°C.

Tantalum capacitors are less desirable than ceramic for use

as output capacitors because they are more expensive when

comparing equivalent capacitance and voltage ratings in the

0.47

μF to 4.7 μF range. Another important consideration is

that tantalum capacitors have higher ESR values than equiv-

alent size ceramics. This means that while it may be possible

to find a tantalum capacitor with an ESR value within the sta-

ble range, it would have to be larger in capacitance (which

means bigger and more costly) than a ceramic capacitor with

the same ESR value. It should also be noted that the ESR of

a typical tantalum will increase about 2:1 as the temperature

goes from 25°C down to −40°C, so some guard band must

be allowed.

Capacitor

Min Value

Unit

Description

Recommended Type

CLDO1

0.47

µF

LDO1 output capacitor Ceramic, 6.3V, X5R

CLDO2

0.47

µF

LDO2 output capacitor Ceramic, 6.3V, X5R

CSW1

µF

SW1 output capacitor

Ceramic, 6.3V, X5R

CSW2

µF

SW2 output capacitor

Ceramic, 6.3V, X5R

LM26480 Datasheet(PDF) 25 Page - National Semiconductor (TI)

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