CS5201-3GST3 datasheet(5/6 Pages) CHERRY

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Applications Information: continued

Figure 2. Conductor parasitic resistance effects can be minimized with

the above grounding scheme for fixed output regulators.

The CS5201-3 linear regulator includes thermal shutdown

and current limit circuitry to protect the device. High

power regulators such as these usually operate at high

junction temperatures so it is important to calculate the

power dissipation and junction temperatures accurately to

ensure that an adequate heat sink is used.

The case is connected to VOUT on the CS5201-3, and electri-

cal isolation may be required for some applications.

Thermal compound should always be used with high cur-

rent regulators 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 3.5ûC/W. For a high current regula-

tor such as the CS5201-3 the majority of the heat is generat-

ed in the power transistor section. The value for RQSA

depends on the heat sink type, while RQCS depends on fac-

tors such as package type, heat sink interface (is an insula-

tor and thermal grease used?), and the contact area

between the heat sink and the package. Once these calcula-

tions 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 applica-

tion note ÒThermal Management for Linear Regulators.Ó

Calculating Power Dissipation and Heat Sink Requirements

VOUT

VIN

CS5201-3

VIN

RLOAD

conductor

parasitic resistance

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