lp3856et-adj National Semiconductor Corporation, lp3856et-adj Datasheet - Page 13

• Manufacturer Part Number: lp3856et-adj
• Description: 3a Fast Response Ultra Low Dropout Linear Regulators
• Manufacturer: National Semiconductor Corporation

Application Hints
possible conditions, the junction temperature must be within
the range specified under operating conditions. The total
power dissipation of the device is given by:
P
where I
(specified under Electrical Characteristics).
The maximum allowable temperature rise (T
on the maximum ambient temperature (T
cation, and the maximum allowable junction temperature
(T
T
The maximum allowable value for junction to ambient Ther-
mal Resistance, θ
θ
LP3856-ADJ is available in TO-220 and TO-263 packages.
The thermal resistance depends on amount of copper area
or heat sink, and on air flow. If the maximum allowable value
of θ
and ≥ 60 ˚C/W for TO-263 package no heatsink is needed
since the package can dissipate enough heat to satisfy these
requirements. If the value for allowable θ
limits, a heat sink is required.
HEATSINKING TO-220 PACKAGE
The thermal resistance of a TO220 package can be reduced
by attaching it to a heat sink or a copper plane on a PC
board. If a copper plane is to be used, the values of θ
be same as shown in next section for TO263 package.
The heatsink to be used in the application should have a
heatsink to ambient thermal resistance,
θ
In this equation, θ
to the surface of the heat sink and θ
tance from the junction to the surface of the case. θ
about 3˚C/W for a TO220 package. The value for θ
pends on method of attachment, insulator, etc. θ
between 1.5˚C/W to 2.5˚C/W. If the exact value is unknown,
2˚C/W can be assumed.
HEATSINKING TO-263 PACKAGE
The TO-263 package uses the copper plane on the PCB as
a heatsink. The tab of these packages are soldered to the
copper plane for heat sinking. Figure 3 shows a curve for the
θ
a typical PCB with 1 ounce copper and no solder mask over
the copper area for heat sinking.
JA
HA
JA
Rmax
D
Jmax
= (V
≤ θ
of TO-263 package for different copper area sizes, using
= T
JA
):
= T
JA
calculated above is ≥ 60 ˚C/W for TO-220 package
Rmax
IN
GND
−V
− θ
Jmax
OUT
/ P
is the operating ground current of the device
CH
− T
D
)I
− θ
OUT
Amax
CH
JA
JC
, can be calculated using the formula:
is the thermal resistance from the case
+ (V
.
IN
)I
GND
(Continued)
JC
is the thermal resis-
JA
Amax
falls below these
Rmax
) of the appli-
) depends
CH
CH
varies
JA
JC
de-
will
is
13
As shown in the figure, increasing the copper area beyond 1
square inch produces very little improvement. The minimum
value for θ
32˚C/W.
Figure 4 shows the maximum allowable power dissipation
for TO-263 packages for different ambient temperatures,
assuming θ
ture is 125˚C.
FIGURE 3. θ
FIGURE 4. Maximum power dissipation vs ambient
JA
JA
temperature for TO-263 package
for the TO-263 package mounted to a PCB is
is 35˚C/W and the maximum junction tempera-
JA
vs Copper (1 Ounce) Area for TO-263
package
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