SC561

12

Thermal Considerations

Although each of the two LDOs in the SC561 can provide

300mA of output current, the maximum power dissipation

in the device is restricted by the miniature package size.

The graphs in Figures 3 and 4 can be used as a guideline

to determine whether the input voltage, output voltages,

output currents, and ambient temperature of the system

result in power dissipation within the operating limits are

met or if further thermal relief is required.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

33.5

4

4.5

5

5.5

6

Input Voltage (V)

Figure 3 — Safe Operating Limit

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

-40

-20

0

20

40

60

80

100

Figure 4 — Maximum P

The following procedure can be followed to determine if

the thermal design of the system is adequate. The junc-

tion temperature of the SC561 can be determined in

Applications Information (continued)

known operating conditions using the following

equation:

T

J

= T

A

+(P

D

x θ

JA

)

where

T

J

= Junction Temperature (°C)

T

A

= Ambient Temperature (°C)

P

D

= Power Dissipation (W)

θ

JA

= Thermal Resistance Junction to Ambient (°C/W)

Example

A SC561 is used to provide outputs of 3.1V, 150mA from

LDOA and 2.85V, 250mA from LDOB. The input voltage is

4.2V, and the ambient temperature of the system is 40°C.

P

D

= 0.15(4.2 – 3.1) + 0.25(4.2 – 2.85)

= 0.503W

and

T

J

= 40 + (0.503 x 157) = 118.9°C

Figure 4 shows that the junction temperature would be

within the maximum specification of 150°C for this power

dissipation. This means that operation of the SC561 under

these conditions is within the specified limits and the

device would not require further thermal relief measures.