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CS5253-1GDP5

3.0 A LDO 5−Pin Adjustable Linear Regulator

ON Semiconductor 

CS5253−1

Current Limit

The internal current limit circuit limits the output current

under excessive load conditions.

Short Circuit Protection

The device includes short circuit protection circuitry that

clamps the output current at approximately 500 mA less than

its current limit value. This provides for a current foldback

function, which reduces power dissipation under a direct

shorted load.

Thermal Shutdown

The thermal shutdown circuitry is guaranteed by design to

activate above a die junction temperature of approximately

150°C and to shut down the regulator output. This circuitry

has 25°C of typical hysteresis, thereby allowing the

regulator to recover from a thermal fault automatically.

Calculating Power Dissipation and Heat Sink

Requirements

High power regulators such as the CS5253−1 usually

operate at high junction temperatures. Therefore, it is

important to calculate the power dissipation and junction

temperatures accurately to ensure that an adequate heat sink

is used. Since the package tab is connected to VOUT on the

CS5253−1, electrical isolation may be required for some

applications. Also, as with all high power packages, thermal

compound in necessary to ensure proper heat flow. For

added safety, this high current LDO includes an internal

thermal shutdown circuit

The thermal characteristics of an IC depend on the

following four factors: junction temperature, ambient

temperature, die power dissipation, and the thermal

resistance from the die junction−to−ambient air. The

maximum junction temperature can be determined by:

TJ(max) + TA(max) ) PD(max) RqJA

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

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 which is measured in degrees per watt. Like series

electrical resistances, these thermal resistances are summed

to determine the total thermal resistance between the die

junction and the surrounding air, RqJA. This total thermal

resistance is comprised of three components. These resistive

terms are measured from junction to case (RqJC), case to heat

sink (RqCS), and heat sink to ambient air (RqSA). The

equation is:

RqJA + RqJC ) RqCS ) RqSA

The value for RqJC is 2.5°C/watt for the CS5253−1 in the

D2PAK−5 package. For a high current regulator such as the

CS5253−1 the majority of heat is generated in the power

transistor section. The value for RqSA depends on the heat

sink type, while the RqCS depends on factors such as

package type, heat sink interface (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 our application note “Thermal

Management,” document number AND8036/D, available

through the Literature Distribution Center or via our website

at http://www.onsemi.com.

http://onsemi.com

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