SC1458

10

Applications Information (continued)

until the fault is removed and the load current returns to

a specified level.

Component Selection

A capacitance of 1μF or larger on each output is

recommended to ensure stability. Ceramic capacitors

of type X5R or X7R should be used because of their

low ESR and stable temperature coefficients. It is also

recommended that the input be bypassed with a 2.2μF,

low ESR X5R or X7R capacitor to minimize noise and

improve transient response. Note: Tantalum and Y5V

capacitors are not recommended.

A

bypass capacitor (minimum of 22nF) should be

connected between the BYP and GND pins to meet all

noise-sensitive requirements. Increasing the capacitance

to 100nF will further improve PSRR and output noise

(SC1458B only).

Thermal Considerations

Although each of the two LDOs in the SC1458 can

provide 300mA of output current, the maximum power

dissipation in the device is restricted by the miniature

package size. The graphs in Figure 2 and Figure 3 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 or if further thermal relief is

required.

0

0.1

0.2

0.3

0.4

0.5

0.6

3

3.5

4

4.5

5

5.5

6

Input Voltage (V)

Figure 2 — Safe Operating Limit

0

0.3

0.6

0.9

1.2

1.5

1.8

-40

-20

0

20

40

60

80

100

Ambient Temperature (

Figure 3 — Maximum P

the thermal design of the system is adequate.The junction

temperature of the SC1458 can be determined in 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

An SC1458A is used to provide outputs of 2.5V, 150mA

from LDOA and 1.8V, 200mA from LDOB.

The input

voltage is 4.2V, and the ambient temperature of the

system is 60°C.

P

D

= 0.15(4.2 – 2.5) + 0.2(4.2 – 1.8)

= 0.74W

and

T

J

= 60 + (0.74 x 50) = 97°C

Figure 3 shows that the power dissipation is within limits

at T

A

= 60°C and calculation of T

J

shows that it is within the

specified limit of 150°C.

This means that operation of the SC1458 under these

conditions is within the specified limits and the device

would not require further thermal relief measures.