Application Hints

Input Capacitors

Although not always required, an input capacitor is recom-

mended. A supply bypass capacitor on the input assures that

the reference is working from a source with low impedance,

which improves stability. A bypass capacitor can also im-

prove transient response by providing a reservoir of stored

energy that the reference can utilize in case where the load

current demand suddenly increases. The value used for C

IN

may be used without limit. Refer to the typical application

section for examples of input capacitors.

Output Capacitors

The LM4140 requires a 1µF (nominally) output capacitor for

loop stability (compensation) as well as transient response.

During the sudden changes in load current demand, the out-

put capacitor must source or sink current during the time it

takes the control loop of the LM4140 to respond.

This capacitor must be selected to meet the requirements of

minimum capacitance and equivalent series resistance

(ESR) range.

In general, the capacitor value must be at least 0.2µF (over

the actual ambient operating temperature), and the ESR

must be within the range indicated in

Figure 1, Figure 2 and

Figure 3.

Tantalum Capacitors

Surface-mountable solid tantalum capacitors offer a good

combination of small physical size for the capacitance value,

and ESR in the range needed for by the LM4140. The results

of testing the LM4140 stability with surface mount solid tan-

talum capacitors show good stability with values in the range

of 0.1µF. However, optimum performance is achieved with a

1µF capacitor.

Tantalum capacitors that have been verified as suitable for

use with the LM4140 are shown in

Table 1.

TABLE 1. Surface-Mount Tantalum Capacitor Selection

Guide

1µF Surface-Mount Tantalums

Manufacturer

Part Number

Kemet

T491A105M010AS

NEC

NRU105N10

Siemens

B45196-E3105-K

Nichicon

F931C105MA

Sprague

293D105X0016A2T

2.2µF Surface-Mount Tantalums

Kemet

T491A225M010AS

NEC

NRU225M06

Siemens

B45196/2.2/10/10

Nichicon

F930J225MA

Sprague

293D225X0010A2T

Aluminum Electrolytic Capacitors

Although probably not a good choice for a production design,

because of relatively large physical size, an aluminium elec-

trolytic capacitor can be used in the design prototype for an

LM4140 reference. A 1µF capacitor meeting the ESR condi-

tions can be used. If the operating temperature drops below

0˚C, the reference may not remain stable, as the ESR of the

aluminium electrolytic capacitor will increase, and may ex-

ceed the limits indicated in the figures.

Multilayer Ceramic Capacitors

Surface-mountable multilayer ceramic capacitors may be an

attractive choice because of their relatively small physical

size and excellent RF characteristics.

DS101079-28

FIGURE 1. 0.22 µF ESR Range

DS101079-29

FIGURE 2. 1 µF ESR Range

DS101079-30

FIGURE 3. 10 µF ESR Range

www.national.com

9