Datasheet

T
HA
d T
JA
- (T
CH
+ T
JC
)
d
'T
J
P
D
T
JA
'T
J
= T
J(MAX)
- T
A(MAX)
LP38856
www.ti.com SNVS336E –JUNE 2006–REVISED APRIL 2013
POWER DISSIPATION AND HEAT-SINKING
A heat-sink may be required depending on the maximum power dissipation and maximum ambient temperature
of the application. Under all possible conditions, the junction temperature must be within the range specified
under operating conditions.
The total power dissipation of the device is the sum of three different points of dissipation in the device.
The first part is the power that is dissipated in the NMOS pass element, and can be determined with the formula:
P
D(PASS)
= (V
IN
- V
OUT
) × I
OUT
(1)
The second part is the power that is dissipated in the bias and control circuitry, and can be determined with the
formula:
P
D(BIAS)
= V
BIAS
× I
GND(BIAS)
where
• I
GND(BIAS)
is the portion of the operating ground current of the device that is related to V
BIAS
. (2)
The third part is the power that is dissipated in portions of the output stage circuitry, and can be determined with
the formula:
P
D(IN)
= V
IN
× I
GND(IN)
where
• I
GND(IN)
is the portion of the operating ground current of the device that is related to V
IN
. (3)
The total power dissipation is then:
P
D
= P
D(PASS)
+ P
D(BIAS)
+ P
D(IN)
(4)
The maximum allowable junction temperature rise (ΔT
J
) depends on the maximum anticipated ambient
temperature (T
A(MAX)
) for the application, and the maximum allowable operating junction temperature (T
J(MAX)
):
(5)
The maximum allowable value for junction to ambient Thermal Resistance, θ
JA
, can be calculated using the
formula:
(6)
The LP38856 is available in TO-220 and DDPAK/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
JA
calculated above is ≥ 60 °C/W for TO-220 package and ≥ 60 °C/W for DDPAK/TO-263 package no heat-sink is
needed since the package alone can dissipate enough heat to satisfy these requirements. If the value needed for
allowable θ
JA
falls below these limits, a heat-sink is required.
Heat-Sinking The TO-220 Package
The TO-220 package has a θ
JA
rating of 60°C/W, and a θ
JC
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
JA
will be same as shown in next section for
DDPAK/TO-263 package.
The heat-sink to be used in the application should have a heat-sink to ambient thermal resistance, θ
HA
:
where
• θ
JA
is the required total thermal resistance from the junction to the ambient air
• θ
CH
is the thermal resistance from the case to the surface of the heat sink
• θ
JC
is the thermal resistance from the junction to the surface of the case. (7)
For this equation, θ
JC
is about 3°C/W for a TO-220 package. The value for θ
CH
depends on method of
attachment, insulator, etc. θ
CH
varies between 1.5°C/W to 2.5°C/W. Consult the heat-sink manufacturer
datasheet for details and recommendations.
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