CS5201-3GDPR3G datasheet(5/8 Pages) ONSEMI

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CS5201−3

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Output Voltage Sensing

Since the CS5201−3 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. For best results the

regulator should be connected as shown in Figure 10.

Figure 10. Conductor Parasitic Resistance Effects

Can Be Minimized With the Above Grounding

Scheme For Fixed Output Regulators

VIN

VOUT

CS5201−3

Conductor Parasitic

Resistance

RLOAD

Calculating Power Dissipation and Heatsink

Requirements

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 heatsink is used.

The case is connected to VOUT on the CS5201−3,

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 3.5°C/W for a given package type

based on an average die size. For a high current regulator

such as the CS5201−3 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.

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