LP38856-1.2EVAL National Semiconductor, LP38856-1.2EVAL Datasheet - Page 11

• Manufacturer Part Number: LP38856-1.2EVAL
• Description: BOARD EVALUATION LP38856-1.2
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LP38856-1.2EVAL

• Channels Per Ic: 1 - Single
• Voltage - Output: 1.2V
• Current - Output: 3A
• Voltage - Input: 3 ~ 5.5V
• Regulator Type: Positive Fixed
• Operating Temperature: -40°C ~ 125°C
• Board Type: Fully Populated
• Utilized Ic / Part: LP38856
• Lead Free Status / RoHS Status: Not applicable / Not applicable

where I
of the device that is related to V
The third part is the power that is dissipated in portions of the
output stage circuitry, and can be determined with the formu-
la:
where I
of the device that is related to V
The total power dissipation is then:
The maximum allowable junction temperature rise (ΔT
pends on the maximum anticipated ambient temperature (T
(MAX)
ating junction temperature (T
The maximum allowable value for junction to ambient Ther-
mal Resistance, θ
The LP38856 is available in TO-220 and TO-263 packages.
The thermal resistance in the application depends on amount
of copper area or heat-sink, and on air flow. If the maximum
allowable value of θ
TO-220 package and
sink is needed since the package alone can dissipate enough
heat to satisfy these requirements. If the value needed for al-
lowable θ
Heat-Sinking The TO-220 Package
The TO-220 package has a θ
rating of 3°C/W. These ratings are for the package only, no
additional heat-sinking, and with no airflow.
The thermal resistance of a TO-220 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
as shown in next section for TO-263 package.
The heat-sink to be used in the application should have a
heat-sink to ambient thermal resistance, θ
where θ
junction to the ambient air, θ
the case to the surface of the heat sink, and θ
resistance from the junction to the surface of the case.
For this equation, θ
The value for θ
tor, etc. θ
heat-sink manufacturer datasheet for details and recommen-
dations.
Heat-Sinking The TO-263 Package
The TO-263 package has a θ
rating of 3°C/W. These ratings are for the package only, no
additional heat-sinking, and with no airflow.
) for the application, and the maximum allowable oper-
GND(BIAS)
GND(IN)
JA
CH
JA
is the required total thermal resistance from the
P
falls below these limits, a heat-sink is required.
varies between 1.5°C/W to 2.5°C/W. Consult the
D
P
is the portion of the operating ground current
= P
CH
D(BIAS)
is the portion of the operating ground current
P
JA
depends on method of attachment, insula-
D(PASS)
D(IN)
JC
, can be calculated using the formula:
JA
≥
= V
is about 3°C/W for a TO-220 package.
= V
60 °C/W for TO-263 package no heat-
calculated above is
BIAS
+ P
IN
CH
× I
D(BIAS)
J(MAX)
× I
JA
JA
is the thermal resistance from
GND(IN)
BIAS
IN
GND(BIAS)
rating of 60°C/W, and a θ
rating of 60°C/W, and a θ
.
):
.
+ P
D(IN)
HA
:
JC
≥
JA
is the thermal
60 °C/W for
will be same
J
) de-
(2)
(3)
(4)
(5)
(6)
(7)
JC
JC
A
11
The TO-263 package uses the copper plane on the PCB as
a heat-sink. The tab of this package is soldered to the copper
plane for heat-sinking. The graph below shows a curve for the
θ
a typical PCB with 1 ounce copper and no solder mask over
the copper area for heat-sinking.
As shown in Figure 1, increasing the copper area beyond 1
square inch produces very little improvement. The minimum
value for θ
32°C/W.
Figure 2 shows the maximum allowable power dissipation for
TO-263 packages for different ambient temperatures, assum-
ing θ
125°C.
FIGURE 1. θ
JA
FIGURE 2. Maximum Power Dissipation vs Ambient
of TO-263 package for different copper area sizes, using
JA
is 35°C/W and the maximum junction temperature is
JA
Temperature for TO-263 package
JA
for the TO-263 package mounted to a PCB is
vs Copper (1 Ounce) Area for the TO-263
package
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