LP2953_15 datasheet(32/41 Pages) TI

32 / 41 page

LP2952-N, LP2952A, LP2953, LP2953A

SNVS095F – MAY 2004 – REVISED MARCH 2015

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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 (RθJA) for the specific application.

The industrial temperature range (

−40°C ≤ TJ ≤ 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 RθJA = 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. PTOTAL = (VIN − VOUT) IOUT + (VIN) IG

Current/Voltage Diagram

The next parameter which must be calculated is the maximum allowable temperature rise, TR(MAX). This is

calculated by using the formula:

TR(MAX) = TJ(MAX) − TA(MAX) = RθJA × P(MAX)

where:

•

TJ(MAX) is the maximum allowable junction temperature

•

TA(MAX) is the maximum ambient temperature

(3)

Using the calculated values for TR(MAX) and P(MAX), the required value for junction-to-ambient thermal resistance,

RθJA, can now be found.

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