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LP38856 Datasheet(PDF) 11 Page - Texas Instruments

Part No. LP38856
Description  3A Fast-Response High-Accuracy LDO Linear Regulator with Enable
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LP38856 Datasheet(HTML) 11 Page - Texas Instruments

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THA d TJA - (TCH + TJC)
d
'
TJ
PD
TJA
'TJ = TJ(MAX) - TA(MAX)
LP38856
www.ti.com
SNVS336D – JUNE 2006 – REVISED OCTOBER 2011
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:
PD(PASS) = (VIN - VOUT) × IOUT
(1)
The second part is the power that is dissipated in the bias and control circuitry, and can be determined with the
formula:
PD(BIAS) = VBIAS × IGND(BIAS)
where
IGND(BIAS) is the portion of the operating ground current of the device that is related to VBIAS.
(2)
The third part is the power that is dissipated in portions of the output stage circuitry, and can be determined with
the formula:
PD(IN) = VIN × IGND(IN)
where
IGND(IN) is the portion of the operating ground current of the device that is related to VIN.
(3)
The total power dissipation is then:
PD = PD(PASS) + PD(BIAS) + PD(IN)
(4)
The maximum allowable junction temperature rise (
ΔTJ) depends on the maximum anticipated ambient
temperature (TA(MAX)) for the application, and the maximum allowable operating junction temperature (TJ(MAX)):
(5)
The maximum allowable value for junction to ambient Thermal Resistance,
θJA, 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
θJA
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
θJA falls below these limits, a heat-sink is required.
Heat-Sinking The TO-220 Package
The TO-220 package has a
θJA rating of 60°C/W, and a θJC 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
θJA 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,
θHA:
where
θJA is the required total thermal resistance from the junction to the ambient air
θCH is the thermal resistance from the case to the surface of the heat sink
θJC is the thermal resistance from the junction to the surface of the case.
(7)
For this equation,
θJC is about 3°C/W for a TO-220 package. The value for θCH depends on method of
attachment, insulator, etc.
θCH varies between 1.5°C/W to 2.5°C/W. Consult the heat-sink manufacturer
datasheet for details and recommendations.
Copyright © 2006–2011, Texas Instruments Incorporated
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