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CS8221YDP3

Micropower 5.0 V, 100 mA Low Dropout Linear Regulator

ON Semiconductor 

CS8221

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) + NJVIN(max) * VOUT(min)NjIOUT(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

application, and

IQ is the quiescent current the regulator consumes at

IOUT(max).

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

permissible value of RΘJA can be calculated:

RQJA

+

150°C *

PD

TA

(2)

The value of RΘJA can then be compared with those in the

package section of the data sheet. Those packages with

RΘJA’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.

IIN

VIN

CS8221

IOUT

VOUT

HEAT SINKS

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 RΘJA.

RQJA + RQJC ) RQCS ) RQSA

(3)

where:

RΘJC = the junction−to−case thermal resistance,

RΘCS = the case−to−heatsink thermal resistance, and

RΘSA = the heatsink−to−ambient thermal resistance.

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

RΘJA, it too is a function of package type. RΘCS and RΘSA

are functions of the package type, heatsink and the interface

between them. These values appear in heat sink data sheets

of heat sink manufacturers.

IQ

Figure 5. Single Output Regulator With Key

Performance Parameters Labeled

http://onsemi.com

5

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