ML7818A(2005) 查看數據表（PDF） - Micro Electronics
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ML7818A Datasheet PDF : 11 Pages
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( 4 ) Thermal Design
The heat radiation balance model of the TO-220 with the heat sink is shown as follows.
Heat radiation balance
Tj = PLOSS X (θjc +θCH + θHS) + Ta
PLOSS = Vin X IQ + (Vin-Vout) X Iout
(℃ )
(4-1)
(W)
(4-2)
Substituting "Eq.(4-2) into "Eq.(4-1)" obtains
Tj = [Vin X IQ +(Vin-Vout) X Iout] X (θjc +θCH +θHS)+Ta
(℃)
(4-3)
In Eq.(4-3)
Vin, Iout, θCH, θHS, Ta depand on using condition.
Tj, IQ,Vout,θjc depend on IC depend on IC specification.
WhenθCH, IQ and Tj are assumed the following values,
Eq.(4-3) becomes Eq.(4-4).
θCH=0.3 to 0.4 (℃/W) Insert the mica paper (0.1t) and thermal conduction silicon grease between
the IC and heat sink and tighten them with the bolt by 4Kg*cm-min.
IQ = 5 to 6mA (max.)
Tj = 125℃ (max.)
Tj(max) = 125 = [5 X Vin + (Vin-Vout) X Iout] X (5+0.3+ θHS) +Ta
(℃)
(4-4)
When fix the Vout, Tj depends on the Vin, Iout, θHS and Ta.
It means;
Lower Vin and / or Iout are required to linit the temperature rise.
Smaller θHS is required for the effective heat reduce (i.e. using the large heat sink).
In the thermal design, when fix the Vin, Iout and Ta, selectthe heat sink which θHS is smaller that
the result of Eq.(4-4).
For more detail, please refer the heat resistance value mentioned in the specification of the heat sink
supplier.
Page 11 of 11
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