RT9199
10
DS9199-11 April 2014www.richtek.com
©
Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Thermal Consideration
RT9199 regulators have internal thermal limiting circuitry
designed to protect the device during overload conditions.
For continued operation, do not exceed absolute maximum
operation junction temperature 125°C. The power
dissipation definition in device is :
P
D
= (V
IN
- V
OUT
) x I
OUT
+ V
IN
x I
Q
The maximum power dissipation depends on the thermal
resistance of IC package, PCB layout, the rate of
surroundings airflow and temperature difference between
junction to ambient. The maximum power dissipation can
be calculated by following formula :
P
D(MAX)
= (T
J(MAX)
− T
A
) / θ
JA
Where T
J(MAX)
is the maximum operation junction
temperature 125°C, T
A
is the ambient temperature and
the θ
JA
is the junction to ambient thermal resistance. The
junction to ambient thermal resistance for SOP-8 package
(Exposed Pad) is 42.9°C/W, on standard JEDEC 51-7 (4
layers, 2S2P) thermal test board. The maximum power
dissipation at T
A
= 25°C can be calculated by following
formula :
P
D(MAX)
= (125°C − 25°C) / 42.9°C/W = 2.33W
Figure 8 shows the package sectional drawing of SOP-8
(Exposed Pad). Every package has several thermal
dissipation paths. As show in Figure 9, the thermal
resistance equivalent circuit of SOP-8 (Exposed Pad). The
path 2 is the main path due to these materials thermal
conductivity. We define the exposed pad is the case point
of the path 2.
Figure 8. SOP-8 (Exposed Pad) Package Sectional
Drawing
Ambient
Molding Compound
Gold Line
Lead Frame
Die Pad
Case (Exposed Pad)
PCB
Figure 9. Thermal Resistance Equivalent Circuit
Junction
R
DIE
R
DIE-ATTACH
R
DIE-PAD
R
GOLD-LINE
R
LEAD FRAME
Case
(Exposed Pad)
R
PCB
R
PCB
Ambient
R
MOLDING-COMPOUND
path 1
path 2
path 3
