7

Application Notes: continued

ommended value and work towards a less expensive

alternative part for each output.

Step 1:

Place the completed circuit with the tantalum

capacitors of the recommended values in an environmen-

tal chamber at the lowest specified operating temperature

and monitor the outputs with an oscilloscope. A decade

higher ESR of an aluminum capacitor. Leave the decade

box outside the chamber, the small resistance added by

the longer leads is negligible.

Step 2:

With the input voltage at its maximum value,

increase the load current slowly from zero to full load on

the output under observation and look for oscillations on

the output. If no oscillations are observed, the capacitor is

large enough to ensure a stable design under steady state

conditions.

Step 3:

Increase the ESR of the capacitor from zero using

the decade box and vary the load current until oscillations

appear. Record the values of load current and ESR that

cause the greatest oscillation. This represents the worst

case load conditions for the output at low temperature.

Step 4

: Maintain the worst case load conditions set in step

3 and vary the input voltage until the oscillations increase.

This point represents the worst case input voltage condi-

tions.

Step 5:

If the capacitor is adequate, repeat steps 3 and 4

with the next smaller valued capacitor. A smaller capaci-

tor will usually cost less and occupy less board space. If

the output oscillates within the range of expected operat-

ing conditions, repeat steps 3 and 4 with the next larger

standard capacitor value.

Step 6:

Test the load transient response by switching in

various loads at several frequencies to simulate its real

working environment. Vary the ESR to reduce ringing.

Step 7:

Remove the unit from the environmental chamber

and heat the IC with a heat gun. Vary the load current as

instructed in step 5 to test for any oscillations.

Once the minimum capacitor value with the maximum

ESR is found, a safety factor should be added to allow for

the tolerance of the capacitor and any variations in regula-

tor performance. Most good quality aluminum electrolytic

capacitors have a tolerance of ±20% so the minimum value

found should be increased by at least 50% to allow for this

tolerance plus the variation which will occur at low temper-

atures. The ESR of the capacitor should be less than 50% of

the maximum allowable ESR found in step 3 above.

Repeat steps 1 through 7 with the capacitor on the other

The maximum power dissipation for a dual output regu-

lator (Figure 1) is:

(1)

Where

application,

application, and

(2)

the package section of the data sheet. Those packages

will keep the die temperature below 150°C.

In some cases, none of the packages will be sufficient to

dissipate the heat generated by the IC, and an external

heatsink will be required.

Figure 1: Dual output regulator with key performance parameters

labeled.

A heat sink effectively increases the surface area of the

package to improve the flow of heat away from the IC and

into the surrounding air.

Each material in the heat flow path between the IC and

the outside environment will have a thermal resistance.

Like series electrical resistances, these resistances are

(3)

where

are functions of the package type, heatsink and the inter-

face between them. These values appear in heat sink data

sheets of heat sink manufacturers.

Heat Sinks

Smart

Regulator

Control

Features

}

150°C - T

A

Calculating Power Dissipation

in a Dual Output Linear Regulator