US1075CT 查看數據表（PDF） - Unisem

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US1075CT

7.5A LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR

Unisem 

US1075

Thermal Design

∆T=Temperature Rise Above Ambient

The US1075 incorporates an internal thermal shutdown

that protects the device when the junction temperature

exceeds the maximum allowable junction temperature.

Although this device can operate with junction tempera-

tures in the range of 150°C ,it is recommended that the

selected heat sink be chosen such that during maxi-

mum continuous load operation the junction tempera-

ture is kept below this number. The example below

shows the steps in selecting the proper Regulator heat

sink for the worst case current consumption using Intel

200MHz microprocessor as the load .

Assuming the following specifications :

θSA = ∆T

PD

θSA = 81 =11.7 ° C/W

6.9

5) Next , a heat sink with lower θsa than the one calcu-

lated in step 4 must be selected. One way to do this is

to simply look at the graphs of the “Heat Sink Temp

Rise Above the Ambient” vs. the “Power Dissipation” and

select a heat sink that results in lower temperature rise

than the one calculated in previous step. The following

heat sinks from AAVID and Thermaloy meet this crite-

ria.

VIN =5V

VO =3.5V

IOUTMAX =4.6A

TA =35° C

Thermalloy

AAVID

0

6021PB

534202B

Air Flow (LFM)

100

200

300

6021PB 6073PB 6109PB

534202B 507302 575002

400

7141D

576802B

The steps for selecting a proper heat sink to keep the

junction temperature below 135°C is given as :

1) Calculate the maximum power dissipation using :

( ) PD = IO UT× VIN−VO UT

PD =4.6×(5−3.5) =6.9 W

2) Select a package from the Regulator data sheet

and record its junction to case (or Tab) thermal

resistance.

Selecting TO220 package gives us :

θJC =2.7° C/W

3) Assuming that the heat sink is Black Anodized, cal-

culate the maximum Heat sink temperature allowed :

Assume , θcs=0.05°C/W (Heat sink to Case thermal

resistance for Black Anodized)

( ) TS=TJ−PD × θJC + θCS

TS =135−6.9×(2.7+0.05) =116 ° C

4) With the maximum heat sink temperature calcu-

lated in the previous step, the Heat Sink to Air thermal

resistance (θsa) is calculated by first calculating the

temperature rise above the ambient as follows :

∆T = TS − TA =116−35=81 ° C

Rev. 1.0

9/14/99

2-37

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