LM2937 datasheet(10/24 Pages) TI

10 / 24 page

LM2937

SNVS100E – MARCH 2000 – REVISED APRIL 2013

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If two capacitors are paralleled, the effective ESR is the parallel of the two individual values. The “flatter” ESR of

the Tantalum will keep the effective ESR from rising as quickly at low temperatures.

HEATSINKING

A heatsink may be required 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 Absolute Maximum Ratings.

To determine if a heatsink is required, the power dissipated by the regulator, PD, must be calculated.

The figure below shows the voltages and currents which are present in the circuit, as well as the formula for

calculating the power dissipated in the regulator:

IIN = IL + IG

PD = (VIN − VOUT) IL + (VIN) IG

Figure 25. Power Dissipation 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)

where

•

TJ (max) is the maximum allowable junction temperature, which is 125°C for commercial grade parts

•

TA (max) is the maximum ambient temperature which will be encountered in the application

(1)

Using the calculated values for TR(max) and PD, the maximum allowable value for the junction-to-ambient

thermal resistance,

θ(J−A), can now be found:

θ(J−A) = TR (max)/PD

(2)

IMPORTANT: If the maximum allowable value for

θ(J−A) is found to be ≥ 53°C/W for the TO-220 package, ≥

80°C/W for the DDPAK/TO-263 package, or

≥174°C/W for the SOT-223 package, no heatsink is needed since

the package alone will dissipate enough heat to satisfy these requirements.

If the calculated value for

θ(J−A) falls below these limits, a heatsink is required.

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