LP38513_14 Datasheet, PDF (12/20 Pages) Texas Instruments – 3A Fast Response Ultra Low Dropout Linear Regulator

LP38513
SNVS361C – JULY 2007 – REVISED MARCH 2009
www.ti.com
POWER DISSIPATION/HEAT-SINKING
A heat-sink may be required depending on the maximum power dissipation (P_D(MAX)), maximum ambient
temperature (T_A(MAX))of the application, and the thermal resistance (θ_JA) of the package. Under all possible
conditions, the junction temperature (T_J) must be within the range specified in the Operating Ratings. The total
power dissipation of the device is given by:
P_D = ( (V_IN−V_OUT) x I_OUT) + ((V_IN) x I_GND)
where
• I_GND is the operating ground current of the device (specified under Electrical Characteristics).
(1)
The maximum allowable junction temperature rise (ΔT_J) depends on the maximum expected ambient
temperature (T_A(MAX)) of the application, and the maximum allowable junction temperature (T_J(MAX)):
ΔT_J = T_J(MAX)− T_A(MAX)
(2)
The maximum allowable value for junction to ambient Thermal Resistance, θ_JA, can be calculated using the
formula:
θ_JA = ΔT_J / P_D(MAX)
(3)
LP38513 is available in TO-220 and DDPAk/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_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 can
dissipate enough heat to satisfy these requirements. If the value for allowable θ_JA falls below these limits, a heat
sink is required.
HEAT-SINKING THE TO-220 PACKAGE
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 the same as shown in the next section for
the DDPAK/TO-263 package.
The heatsink to be used in the application should have a heat-sink to ambient thermal resistance,
θ_HA ≤ θ_JA − θ_CH − θ_JC
(4)
In this equation, θ_CH is the thermal resistance from the case to the surface of the heat sink and θ_JC is the thermal
resistance from the junction to the surface of the case. The rated θ_JC is about 3°C/W for a 5-lead TO-22
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. If the exact value is unknown, 2°C/W can be assumed.
HEAT-SINKING THE DDPAK/TO-263 PACKAGE
The DDPAK/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. Figure 25 shows a curve for the θ_JA of DDPAK/TO-263 package for
different copper area sizes, using a typical PCB with 1 ounce copper and no solder mask over the copper area
for heat sinking.
Figure 25. θ_JA vs Copper (1 Ounce) Area for the DDPAK/TO-263 package
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