Datasheet

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SLVS072C − DECEMBER 1992 − REVISED OCTOBER 1995
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
POST OFFICE BOX 1443
• HOUSTON, TEXAS
77251−1443
8−6
THERMAL INFORMATION
The need for smaller surface-mount packages for use on compact printed-wiring boards (PWB) causes an
increasingly difficult problem in the area of thermal dissipation. In order to provide the systems designer with a better
approximation of the junction temperature rise in the thin-shrink small-outline package (TSSOP), the junction-to-lead
thermal resistance (R
θJL
) is provided along with the more typical values of junction-to-ambient and junction-to-case
thermal resistances, R
θJA
and R
θJC
.
R
θJL
is used to calculate the device junction temperature rise measured from the leads of the unit. Consequently, the
junction temperature is dependent upon the board temperature at the leads, R
θJL
, and the internal power dissipation
of the device. The board temperature is contingent upon several variables, including device packing density,
thickness, material, area, and number of interconnects. The R
θJL
value depends on the number of leads connecting
to the die-mount pad, the lead-frame alloy, area of the die, mount material, and mold compound. Since the power level
at which the TSSOP can be used is highly dependent upon both the temperature rise of the PWB and the device itself,
the systems designer can maximize this level by optimizing the circuit board. The junction temperature of the device
can be calculated using the equation T
J
= (P
D
× R
θJL
) + T
L
where T
J
= junction temperature, P
D
= power dissipation,
R
θJL
= junction-to-lead thermal resistance, and T
L
= board temperature at the leads of the unit.
The values of thermal resistance for the TL2218-285 PW are as follows:
Thermal Resistance Typical Junction Rise
R
θJA
151°C/W
R
θJC
31 °C/W
R
θJL
50.5°C/W
TYPICAL CHARACTERISTICS
Table of Graphs
FIGURE
I
O
Output current vs Input voltage 4
V
O
Output voltage vs Input voltage 5
I
O
Output current vs Junction temperature 6
V
O
Output voltage vs Junction temperature 7