CS5203-1GDPR3 Просмотр технического описания (PDF) - Cherry semiconductor

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CS5203-1GDPR3

3A Adjustable Linear Regulator

Cherry semiconductor 

Applications Information: continued

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 3.5û C/W as a single figure for a given

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

current regulator such as the CS5203-1 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

7

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