5

Best load regulation occurs when R1 is connected directly

to the output pin of the regulator as shown in Figure 3. If R1

ratio and the effective resistance between the regulator and

the load becomes

()

Figure 3. Grounding scheme for the adjustable output regulator to min-

imize parasitics.

The CS5207-1 linear regulator includes thermal shutdown

and safe operating area circuitry to protect the device.

High power regulators such as this 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.

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:

These four are related by the equation

(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:

(2)

where

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

junction and the surrounding air.

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

These are connected by the equation:

(3)

result can be substituted in equation (1).

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

high current regulator such as the CS5207-1 the majority of

the heat is generated in the power transistor section. The

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

selected. For further discussion on heat sink selection, see

application note ÒThermal Management for Linear

Regulators.Ó

Calculating Power Dissipation and Heat Sink Requirements

conductor parasitic

resistance

CS5207-1

Adj

R1 + R2

R1

Applications Information: continued