Application Hints

EXTERNAL CAPACITORS

The output capacitor is critical to maintaining regulator stabil-

ity, and must meet the required conditions for both ESR

(Equivalent Series Resistance) and minimum amount of ca-

pacitance.

MINIMUM CAPACITANCE:

The minimum output capacitance required to maintain stabil-

ity is 10 µF (this value may be increased without limit).

Larger values of output capacitance will give improved tran-

sient response.

ESR LIMITS:

The ESR of the output capacitor will cause loop instability if

it is too high or too low. The acceptable range of ESR plotted

versus load current is shown in the graph below.

It is essen-

tial that the output capacitor meet these requirements,

or oscillations can result.

It is important to note that for most capacitors, ESR is speci-

fied only at room temperature. However, the designer must

ensure that the ESR will stay inside the limits shown over the

entire operating temperature range for the design.

For aluminum electrolytic capacitors, ESR will increase by

about 30X as the temperature is reduced from 25˚C to

−40˚C. This type of capacitor is not well-suited for low tem-

perature operation.

Solid tantalum capacitors have a more stable ESR over tem-

perature, but are more expensive than aluminum electrolyt-

ics. A cost-effective approach sometimes used is to parallel

an aluminum electrolytic with a solid Tantalum, with the total

capacitance split about 75/25% with the Aluminum being the

larger value.

If two capacitors are paralleled, the effective ESR is the par-

allel of the two individual values. The “flatter” ESR of the Tan-

talum will keep the effective ESR from rising as quickly at low

temperatures.

HEATSINKING

A heatsink may be required depending on the maximum

power dissipation and maximum ambient temperature of the

application. Under all possible operating conditions, the junc-

tion temperature must be within the range specified under

Absolute Maximum Ratings.

To determine if a heatsink is required, the power dissipated

by the regulator, P

The figure below shows the voltages and currents which are

present in the circuit, as well as the formula for calculating

the power dissipated in the regulator:

The next parameter which must be calculated is the maxi-

mum allowable temperature rise, T

lated by using the formula:

T

where: T

is the maximum allowable junction tem-

perature, which is 125˚C for commercial

grade parts.

T

is the maximum ambient temperature

which will be encountered in the applica-

tion.

Using the calculated values for T

mum allowable value for the junction-to-ambient thermal re-

sistance,

θ

θ

IMPORTANT: If the maximum allowable value for

θ

found to be

≥ 53˚C/W for the TO-220 package, ≥ 80˚C/W for

the TO-263 package, or

≥174˚C/W for the SOT-223 pack-

age, no heatsink is needed since the package alone will dis-

sipate enough heat to satisfy these requirements.

If the calculated value for

θ

heatsink is required.

HEATSINKING TO-220 PACKAGE PARTS

The TO-220 can be attached to a typical heatsink, or se-

cured to a copper plane on a PC board. If a copper plane is

to be used, the values of

θ

the next section for the TO-263.

If a manufactured heatsink is to be selected, the value of

heatsink-to-ambient thermal resistance,

θ

calculated:

θ

Where:

θ

(J−C)

is defined as the thermal resistance from

the junction to the surface of the case. A

value of 3˚C/W can be assumed for

θ

(J−C)

for this calculation.

θ

(C−H)

is defined as the thermal resistance be-

tween the case and the surface of the heat-

sink. The value of

θ

(C−H)

will vary from

about 1.5˚C/W to about 2.5˚C/W (depend-

ing on method of attachment, insulator,

etc.). If the exact value is unknown, 2˚C/W

should be assumed for

θ

When a value for

θ

a heatsink must be selected that has a value that is less than

or equal to this number.

Output Capacitor ESR

DS011280-24

FIGURE 1. ESR Limits

DS011280-27

FIGURE 2. Power Dissipation Diagram

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