ML7900 datasheet(10/10 Pages) MICRO-ELECTRONICS

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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

(℃ )

(4-1)

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

(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)

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;

For more detail, please refer the heat resistance value mentioned in the specification of the heat sink

supplier.

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.

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).

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