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CS5207-1GT3

7.0 A Adjustable Linear Regulator

ON Semiconductor 

CS5207–1

Output Voltage Sensing

Since the CS5207–1 is a three terminal regulator, it is not

possible to provide true remote load sensing. Load

regulation is limited by the resistance of the conductors

connecting the regulator to the load.

Best load regulation occurs when R1 is connected directly

to the output pin of the regulator as shown in Figure 9. If R1

is connected to the load, RC is multiplied by the divider ratio

and the effective resistance between the regulator and the

load becomes.

ǒ Ǔ RC

R1 ) R2

R1

where RC = conductor parasitic resistance.

VIN

VIN

VOUT

Conductor Parasitic

RC

Resistance

CS5207–1

R1

Adj

RLOAD

R2

Figure 9. Grounding Scheme for Adjustable Output

Regulator to Minimize Parasitic Resistance Effects

Calculating Power Dissipation and Heat Sink

Requirements

The CS5207–1 linear regulator includes thermal

shutdown and safe operating area 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 CS5207–1,

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 RΘJA (°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 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 RΘJA, the total thermal resistance between the

junction and the surrounding air.

1. Thermal Resistance of the junction to case, RΘJC

(°C/W)

2. Thermal Resistance of the case to Heat Sink, RΘCS

(°C/W)

3. Thermal Resistance of the Heat Sink to the ambient

air, RΘSA (°C/W)

These are connected by the equation:

RQJA + RQJC ) RQCS ) RQSA

(3)

The value for RΘJA is calculated using equation (3) and

the result can be substituted in equation (1).

The value for RΘJC is normally quoted as a single figure

for a given package type based on 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

value for RΘSA depends on the heat sink type, while RΘCS

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 RΘJA can be calculated and the proper heat sink selected.

For further discussion on heat sink selection, see application

note “Thermal Management for Linear Regulators,”

document number SR006AN/D, available through the

Literature Distribution Center or via our website at

http://onsemi.com.

http://onsemi.com

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