LP2957ISXSLASHNOPB datasheet(10/24 Pages) TI

10 / 24 page

LP2957, LP2957A

SNVS102C – JUNE 1998 – REVISED APRIL 2013

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HEATSINK REQUIREMENTS

A heatsink may be required with the LP2957 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 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 (since this produces the maximum DC input

voltage to the regulator), and the maximum load current must also be used. Figure 30 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 30.

PTOTAL = (VIN − 5)I L + (VIN)IG

*See EXTERNAL CAPACITORS

Figure 30. Basic 5V Regulator Circuit

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) − T A(Max)

where

•

TJ(Max) is the maximum allowable junction temperature

•

TA(Max) is the maximum ambient temperature

(1)

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

resistance,

θ (JA), can now be found:

θ(JA) = TR(Max)/P(Max)

(2)

If the calculated value is 60°C/W or higher , the regulator may be operated without an external heatsink. If the

calculated value is below 60°C/W, an external heatsink is required. The required thermal resistance for this

heatsink,

θ(HA), can be calculated using the formula:

θ(HA) = θ(JA) − θ (JC) − θ(CH)

where

•

θ(JC) is the junction-to-case thermal resistance, which is specified as 3°C/W for the LP2957

•

θ(CH) is the case-to-heatsink thermal resistance, which is dependent on the interfacing material

(see Table 1 and Table 2)

(3)

Typical TO-220 Case-To-Heatsink

Thermal Resistance in °C/W

Table 1. (From AAVID)

Silicone Grease

1.0

Dry Interface

1.3

Mica with Grease

1.4

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