CS8321

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4

recommended value and work towards a less expensive

alternative part.

Step 1: Place the completed circuit with a tantalum

capacitor of the recommended value in an environmental

chamber at the lowest specified operating temperature and

monitor the outputs with an oscilloscope. A decade box

connected in series with the capacitor will simulate the

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 while

observing the output for any oscillations. 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 regulator 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

conditions.

Step 5: If the capacitor is adequate, repeat steps 3 and 4

with the next smaller valued capacitor. A smaller capacitor

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

output oscillates within the range of expected operating

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: Raise the temperature to the highest specified

operating temperature. 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 regulator

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

temperatures. The ESR of the capacitor should be less than

50% of the maximum allowable ESR found in step 3 above.

CALCULATING POWER DISSIPATION

IN A SINGLE OUTPUT LINEAR REGULATOR

The maximum power dissipation for a single output

regulator (Figure 5) is:

PD(max) + (VIN(max) * VOUT(min))IOUT(max)

) VIN(max)IQ

(1)

where:

application, and

RqJA +

150°C * TA

PD

(2)

package section of the data sheet. Those packages with

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 5. Single Output Regulator with Key

Performance Parameters Labeled

CS8321

HEATSINKS

A heatsink 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 summed to

RqJA + RqJC ) RqCS ) RqSA

(3)

where:

functions of the package type, heatsink and the interface

between them. These values appear in heatsink data sheets

of heatsink manufacturers.