CS8121 datasheet(7/8 Pages) CHERRY

7 / 8 page

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 temperatures. The ESR of the capacitor should be less

than 50% of the maximum allowable ESR found in step 3

above.

The maximum power dissipation for a single output regu-

lator (Figure 7) is:

PD(max) = {VIN(max) - VOUT(min)}IOUT(max) + VIN(max)IQ

(1)

where:

VIN(max) is the maximum input voltage,

VOUT(min) is the minimum output voltage,

IOUT(max) is the maximum output current for the applica-

tion, and

IQ is the quiescent current the regulator consumes at

IOUT(max).

Once the value of PD(max) is known, the maximum permis-

sible value of RQJA can be calculated:

RQJA =

(2)

The value of RQJA can then be compared with those in

the package section of the data sheet. Those packages

with RQJA's less than the calculated value in equation 2

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.

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

summed to determine the value of RQJA:

RQJA = RQJC + RQCS + RQSA

(3)

where:

RQJC = the junctionÐtoÐcase thermal resistance,

RQCS = the caseÐtoÐheatsink thermal resistance, and

RQSA = the heatsinkÐtoÐambient thermal resistance.

RQJC appears in the package section of the data sheet. Like

RQJA, it too is a function of package type. RQCS and RQSA

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.

150¡C - T

Application Notes: continued

Calculating Power Dissipation

in a Single Output Linear Regulator

VIN

Smart

Regulator

VOUT

IOUT

IIN

Control

Features

}

Figure 7: Single output regulator with key performance parameters

labeled.

RESET

VOUT

RRST

C2**

CRST

ENABLE

VIN

C1*

0.1

RESET

Port

5V to

mP and

System

Power

CS-8121

C1*required if regulator is located far from

the power supply filter.

C2** required for stability.

Heat Sinks

Figure 6: Test and application circuit showing output compensation.

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