CS5201-1GST3 데이터 시트보기 (PDF) - Cherry semiconductor

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CS5201-1GST3

1A Adjustable Linear Regulator

Cherry semiconductor 

Applications Information: continued

VIN

C1

IN4002 (optional)

VIN

VOUT

CS5201-1

R1

Adj

VOUT

C2

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)

These four are related by the equation

CAdj

R2

Figure 3. Protection diode for large output capacitors.

Output Voltage Sensing

Since the CS5201-1 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 the adjustable regulator, the best load regulation

occurs when R1 is connected directly to the output pin of

the regulator as shown in Figure 4. 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

( ) RC ´ R1 + R2

R1

RC = conductor parasitic resistance

VIN

VIN

VOUT

conductor parasitic

RC resistance

CS5201-1

R1

Adj

RLOAD

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:

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.

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)

R2

Figure 4. Grounding scheme for the adjustable output regulator to min-

imize parasitic resistance effects.

Calculating Power Dissipation and Heat Sink Requirements

The CS5201-1 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 heat sink is used.

The case is connected to VOUT on the CS5201-1, and electri-

cal isolation may be required for some applications.

Thermal compound should always be used with high cur-

rent regulators such as these.

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 3.5ûC/W for a given package type

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

such as the CS5201-1 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 fac-

tors such as package type, heat sink interface (is an insula-

tor 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

6

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