ML7908A datasheet(8/10 Pages) MICRO-ELECTRONICS

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3. Thermal Design

( 1 )

Heat Producting

(1-1)

PLOSS-1 : heat producting by own operation

PLOSS-1 = Vin X IQ

(1-2)

PLOSS-2 : heat producing by output current and the input-output differential voltage.

Internal power transistor produces the hest mentioned following equation.

PLOSS-2

= (Vin-Vout) x Iout

(W)

Therefore, the total heat producing PLOSS is :

PLOSS

= PLOSS-1 + PLOSS-2

= Vin X IQ + (Vin-Vout) X Iout

(W)

( 2 )

Thermal Resistance

(2-1)

Definition of Thermal Resistance : θ

Thermal resistance (θ ) is a degree of heat radiation mentioned following equation.

= (T1 - T2)/P (℃ /W)

Heat Producing Quantity

: P (W)

Ambient Temperature or case temperature

:T2 (℃ )

Heat Source Temperature

:T1 (℃ )

(2-2)

Thermal resistance of TO-220

θjc :

θja :

REV B

thermal resistance between IC chip (junction point) and ambience.

Input voltage (Vin) and quiescent current (IQ) produce the heat mentioned below equation.

There are two kinds of heat producting (PLOSS-1, PLOSS-2) in three terminal regulator and the sum of

them is total heat producting of IC (PLOSS).

There are two kinds of thermal resistance of TO-220. One is "θjc" for the application with the heat

sink, the other is "θja" for the application without the heat sink.

thermal resistance between IC chip (junction point) and the package back side

contacting with the heat sink.

Input

GND

OUT

Output

Vin

Vout

Iout

P(W)

T1 > T2

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