LP2953_15 Datasheet, PDF (32/41 Pages) Texas Instruments – LP295x Adjustable Micropower Low-Dropout Voltage Regulators

LP2952-N, LP2952A, LP2953, LP2953A
SNVS095F – MAY 2004 – REVISED MARCH 2015
www.ti.com
10.3 Power Dissipation: Heatsink Requirements (Industrial Temperature Range Devices)
The maximum allowable power dissipation for the LP2952 or LP2953 is limited by the maximum operating
junction temperature (125°C) and the external factors that determine how quickly heat flows away from the part:
the ambient temperature and the junction-to-ambient thermal resistance (R_θJA) for the specific application.
The industrial temperature range (−40°C ≤ T_J ≤ 125°C) parts are manufactured in PDIP and surface-mount
packages which contain a copper lead frame that allows heat to be effectively conducted away from the die,
through the ground pins of the IC, and into the copper of the PC board. Details on heatsinking using PC board
copper are covered later.
Figure 47. Power Dissipation vs Ambient Temperature
for R_θJA = 95°C/W
To determine if a heatsink 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 48 shows the voltages and currents which are present in the circuit. The formula
for calculating the power dissipated in the regulator is also shown in Figure 48:
Figure 48. P_TOTAL = (V_IN − V_OUT) I_OUT + (V_IN) I_G
Current/Voltage Diagram
The next parameter which must be calculated is the maximum allowable temperature rise, T_R(MAX). This is
calculated by using the formula:
T_R(MAX) = T_J(MAX) − T_A(MAX) = R_θJA × P_(MAX)
where:
• T_J(MAX) is the maximum allowable junction temperature
• T_A(MAX) is the maximum ambient temperature
(3)
Using the calculated values for T_R(MAX) and P_(MAX), the required value for junction-to-ambient thermal resistance,
R_θJA, can now be found.
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