CS5207-2 查看數據表（PDF） - Cherry semiconductor

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CS5207-2

7A, 1.5V Fixed Linear Regulator

Cherry semiconductor 

Applications Information: continued

Output Voltage Sensing

Since the CS5207-2 is a three terminal regulator, it is not

possible to provide true remote load sensing. Load regula-

tion is limited by the resistance of the conductors connect-

ing the regulator to the load. For best results the fixed reg-

ulators should be connected as shown in Figure 2.

VIN

VIN

VOUT

conductor parasitic

RC resistance

CS5207-2

RLOAD

Gnd

PD(max)={VIN(max)ÐVOUT(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 RQJA, the total thermal resistance between the

junction and the surrounding air.

Figure 2. Conductor parasitic resistance effects can be minimized with

the above grounding scheme for fixed output regulators.

1. Thermal Resistance of the junction to case, RQJC (¡C/W)

2. Thermal Resistance of the case to Heat Sink, RQCS (¡C/W)

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

RQSA (¡C/W)

These are connected by the equation:

Calculating Power Dissipation and Heat Sink Requirements

The CS5207-2 includes thermal shutdown and safe operat-

ing area circuitry to protect the device. High power regu-

lators such as these usually operate at high junction tem-

peratures so it is important to calculate the power dissipa-

tion and junction temperatures accurately to ensure that an

adequate heat sink is used.

The case is connected to VOUT on the CS5207-2, electrical

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:

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)

RQJA = RQJC + RQCS + RQSA

(3)

The value for RQJA is calculated using equation (3) and the

result can be substituted in equation (1).

RQJC is 1.6¡C/Watt for the CS5207-2. For a high current

regulator such as the CS5207-2 the majority of the heat is

generated in the power transistor section. The value for

RQSA depends on the heat sink type, while 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 calcula-

tions 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 applica-

tion note ÒThermal Management for Linear Regulators.Ó

These four are related by the equation

TJ = TA + PD ´ RQJA

(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:

4

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