CS8140 데이터 시트보기 (PDF) - Cherry semiconductor

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CS8140

5V, 500mA Linear Regulator with ENABLE, , and Watchdog RESET

Cherry semiconductor 

Application Notes: continued

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 condi-

tions.

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 stan-

dard 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 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 regu-

lator (Figure 9) 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).

IIN

VIN

Smart

Regulator

}Control

Features

IQ

IOUT

VOUT

Figure 9: Single output regulator with key performance parameters

labeled.

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

sible value of RQJA can be calculated:

RQJA =

150¡C - TA

PD

(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.

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 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 heatsink data

sheets of heatsink manufacturers.

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