SC560B datasheet(14/18 Pages) SEMTECH

14 / 18 page

SC560

Thermal Considerations

Although each of the two LDOs in the SC560 can provide

300mA of output current, the maximum power dissipation

in the device is restricted by the miniature package size.

The graphs in Figure 3 and Figure 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 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 .7

2 .5

3 .5

4 .5

5 .5

______ T

A =+ 25 °C , P D (M A X )= 0 .8 W

- - - - T A=+ 85 °C , P D(MAX)= 0.41W

Vo = 3 .3 V

Input Voltage (V)

Vo = 1 .5 V

Figure 3 — Safe Operating Limit

0 .2

0 .4

0 .6

0 .8

1 .2

1 .4

1 .6

-40

-20

100

Ambient Temperature (

oC)

T J(Max)= 150°C

T J(Max)= 125 °C

Figure 4 — Maximum P

D vs. TA

Applications Information (continued)

The following procedure can be followed to determine if

the thermal design of the system is adequate. The junction

temperature of the SC560 can be determined in known

operating conditions using the following equation:

J = TA +(PD x θJA)

where

J = Junction Temperature (°C)

A = Ambient Temperature (°C)

D = Power Dissipation (W)

JA = Thermal Resistance Junction to Ambient

(°C/W)

Example

An SC560D is used to provide outputs of 2.8V, 150mA from

LDOA and 1.8V, 200mA from LDOB. The input voltage is

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

D= 0.15(4.2 – 2.8) + 0.2(4.2 – 1.8)

= 0.69W

and

J = 40 + (0.69 x 157) = 148.3°C

Figures 3 and 4 show that the junction temperature

would be within the maximum speci cation of 150°C

for this power dissipation. This means that operation of

the SC560 under these conditions is within the speci ed

limits and the device would not require further thermal

relief measures.

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