CS5205-3GDPR3 datasheet(6/7 Pages) CHERRY

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Applications Information: continued

The CS5205-X series of linear regulators includes thermal

shutdown and safe operating area circuitry to protect the

device. High power regulators such as these usually oper-

ate at high junction temperatures so it is important to cal-

culate the power dissipation and junction temperatures

accurately to ensure that an adequate heat sink is used.

The case is connected to VOUT on the CS5205-X, electrical

isolation may be required for some applications. Thermal

compound should always be used with high current regu-

lators such as these.

The thermal characteristics of an IC depend on the follow-

ing four factors:

1. Maximum Ambient Temperature TA (¡C)

2. Power dissipation PD (Watts)

3. Maximum junction temperature TJ (¡C)

4. Thermal resistance junction to ambient RQJA (C/W)

These four are related by the equation

TJ = TA + PD ´ RQJA

(1)

The maximum ambient temperature and the power dissi-

pation are determined by the design while the maximum

junction temperature and the thermal resistance depend

on the manufacturer and the package type.

The maximum power dissipation for a regulator is:

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

(2)

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

IQ is the maximum quiescent current at IOUT(max).

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 has a thermal resistance. Like series

electrical resistances, these resistances are summed to

determine RQJA, the total thermal resistance between the

junction and the surrounding air.

1. Thermal Resistance of the junction to case, RQJC (¡C/W)

2. Thermal Resistance of the case to Heat Sink, RQCS (¡C/W)

3. Thermal Resistance of the Heat Sink to the ambient air,

RQSA (¡C/W)

These are connected by the equation:

RQJA = RQJC + RQCS + RQSA

(3)

The value for RQJA is calculated using equation (3) and the

result can be substituted in equation (1).

The value for RQJC is normally quoted as a single figure for

a given package type based on an average die size. For a

high current regulator such as the CS5205-X the majority of

the heat is generated in the power transistor section. The

value for RQSA depends on the heat sink type, while RQCS

depends on factors such as package type, heat sink inter-

face (is an insulator and thermal grease used?), and the

contact area between the heat sink and the package. Once

these calculations are complete, the maximum permissible

value of RQJA can be calculated and the proper heat sink

selected. For further discussion on heat sink selection, see

application note ÒThermal Management for Linear

Regulators.Ó

Calculating Power Dissipation and Heat Sink Requirements

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