CS5203A-1 データシートの表示（PDF） - ON Semiconductor

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CS5203A-1

3.0 A Adjustable, and Fixed 1.5 V, 3.3 V and 5.0 V Linear Regulators

ON Semiconductor 

CS5203A−1, CS5203A−2, CS5203A−3, CS5203A−5

Calculating Power Dissipation and Heatsink

Requirements

The CS5203A series of linear regulators 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 Heatsink is used.

The case is connected to VOUT on the CS5203A, electrical

isolation may be required for some applications. Thermal

compound should always be used with high current

regulators such as these.

The thermal characteristics of an IC depend on the

following 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

dissipation 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) * VOUT(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 Heatsink 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 Heatsink, RqCS

(°C/W)

3. Thermal Resistance of the Heatsink 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 CS5203A the majority

of the heat is generated in the power transistor section. The

value for RqSA depends on the Heatsink type, while RqCS

depends on factors such as package type, Heatsink interface

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

area between the Heatsink and the package. Once these

calculations are complete, the maximum permissible value

of RqJA can be calculated and the proper Heatsink selected.

For further discussion on Heatsink selection, see application

note “Thermal Management,” document number

AND8036/D, available through the Literature Distribution

Center or via our website at http://onsemi.com.

http://onsemi.com

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