LP2954 Datasheet, PDF (15/28 Pages) National Semiconductor (TI) – 5V and Adjustable Micropower Low-Dropout Voltage Regulators

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LP2954, LP2954A
SNVS096E – JUNE 1999 – REVISED JULY 2016
8.2.2.4 Power Dissipation
Knowing the device power dissipation and proper sizing of the thermal plane connected to the tab or pad is
critical to ensuring reliable operation. Device power dissipation depends on input voltage, output voltage, and
load conditions and can be calculated with Equation 3.
P_D(MAX) = (V_IN(MAX) – V_OUT) × I_OUT
(3)
Power dissipation can be minimized, and greater efficiency can be achieved, by using the lowest available
voltage drop option that would still be greater than the dropout voltage (V_DO). However, keep in mind that higher
voltage drops result in better dynamic (that is, PSRR and transient) performance.
Power dissipation and junction temperature are most often related by the junction-to-ambient thermal resistance
(R_θJA) of the combined PCB and device package and the temperature of the ambient air (T_A), according to
Equation 4 or Equation 5:
T_J(MAX) = T_A(MAX) + (R_θJA × P_D(MAX))
(4)
P_D(MAX) = (T_J(MAX) – T_A(MAX)) / R_θJA
(5)
Unfortunately, this R_θJA is highly dependent on the heat-spreading capability of the particular PCB design, and
therefore varies according to the total copper area, copper weight, and location of the planes. The R_θJA recorded
in Thermal Information is determined by the specific EIA/JEDEC JESD51-7 standard for PCB and copper-
spreading area, and is to be used only as a relative measure of package thermal performance. For a well-
designed thermal layout, R_θJA is actually the sum of the package junction-to-case (bottom) thermal resistance
(R_θJCbot) plus the thermal resistance contribution by the PCB copper area acting as a heat sink.
8.2.2.5 Estimating Junction Temperature
The EIA/JEDEC standard recommends the use of psi (Ψ) thermal characteristics to estimate the junction
temperatures of surface mount devices on a typical PCB board application. These characteristics are not true
thermal resistance values, but rather package specific thermal characteristics that offer practical and relative
means of estimating junction temperatures. These psi metrics are determined to be significantly independent of
copper-spreading area. The key thermal characteristics (Ψ_JT and Ψ_JB) are given in Thermal Information and are
used in accordance with Equation 6 or Equation 7.
T_J(MAX) = T_TOP + (Ψ_JT × P_D(MAX))
where
• P_D(MAX) is explained in Equation 5
• T_TOP is the temperature measured at the center-top of the device package.
(6)
T_J(MAX) = T_BOARD + (Ψ_JB × P_D(MAX))
where
• P_D(MAX) is explained in Equation 5.
• T_BOARD is the PCB surface temperature measured 1-mm from the device package and centered on the
package edge.
(7)
For more information about the thermal characteristics Ψ_JT and Ψ_JB, Semiconductor and IC Package Thermal
Metrics; for more information about measuring T_TOP and T_BOARD, see Using New Thermal Metrics; and for more
information about the EIA/JEDEC JESD51 PCB used for validating R_θJA, see Thermal Characteristics of Linear
and Logic Packages Using JEDEC PCB Designs. These application notes are available at www.ti.com.
8.2.2.6 Heatsinking the TO-220 Package
A heat sink may be required with the LP2954IT depending on the maximum power dissipation and maximum
ambient temperature of the application. Under all possible operating conditions, the junction temperature must be
within the range specified under Recommended Operating Conditions.
To determine if a heat sink is required, the maximum power dissipated by the regulator, P_(MAX), must be
calculated. It is important to remember that if the regulator is powered from a transformer connected to the AC
line, the maximum specified AC input voltage must be used (because this produces the maximum DC input
voltage to the regulator). Figure 20 shows the voltages and currents that are present in the circuit. The formula
for calculating the power dissipated in the regulator is also shown in Figure 20.
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