RT8278 RICHTEK [Richtek Technology Corporation], RT8278 Datasheet - Page 13

• Manufacturer Part Number: RT8278
• Description: 2A, 24V, 3MHz Step-Down Converter
• Manufacturer: RICHTEK [Richtek Technology Corporation]

Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
P
where T
the ambient temperature, and θ
thermal resistance.
For recommended operating condition specifications of
the RT8278, the maximum junction temperature is 125°C
and T
thermal resistance, θ
(Exposed Pad) packages, the thermal resistance, θ
75°C/W on a standard JEDEC 51-7 four-layer thermal test
board. The maximum power dissipation at T
be calculated by the following formulas :
P
(min. copper area PCB layout)
P
(70mm
The thermal resistance, θ
determined by the package architectural design and the
PCB layout design. The package architectural design is
fixed. However, it's possible to increase thermal
performance via better PCB layout copper design. The
thermal resistance, θ
copper area under the exposed pad of the SOP-8
(Exposed Pad) package.
As shown in Figure 7, the amount of copper area to which
the SOP-8 (Exposed Pad) is mounted on affects thermal
performance. When mounted to the standard SOP-8
(Exposed Pad) (Figure 7a), θ
area under the SOP-8 (Exposed Pad) (Figure 7b) reduces
θ
70mm
The maximum power dissipation depends on operating
ambient temperature for fixed T
DS8278-02 March 2011
JA
D(MAX)
D(MAX)
D(MAX)
to 64°C/W. Further increasing the copper area to
A
2
is the ambient temperature. The junction to ambient
2
= (T
(Figure 7e) will reduce θ
J(MAX)
copper area PCB layout)
= (125°C − 25°C) / (75°C/W) = 1.333W
= (125°C − 25°C) / (49°C/W) = 2.04W
J(MAX)
is the maximum junction temperature, T
− T
A
JA
) / θ
JA
, is layout dependent. For SOP-8
, can be decreased by adding
JA
JA
, of SOP-8 (Exposed Pad) is
JA
JA
is 75°C/W. Adding copper
is the junction to ambient
JA
to 49°C/W.
J (MAX)
and thermal
A
=25°C can
JA
A
, is
is
resistance, θ
curves in Figure 8 allow the designer to see the effect of
rising ambient temperature on the maximum power
dissipation.
Figure 7. Themal Resistance vs. Copper Area Layout
(a) Copper Area = (2.3 x 2.3) mm
(c) Copper Area = 30mm
(d) Copper Area = 50mm
(e) Copper Area = 70mm
(b) Copper Area = 10mm
JA
. For the RT8278 packages, the derating
Design
2
2
2
2
, θ
, θ
, θ
, θ
JA
JA
JA
JA
2
, θ
= 64°C/W
= 54°C/W
= 51°C/W
= 49°C/W
RT8278
JA
www.richtek.com
= 75°C/W
13