LP2954IM datasheet(16/28 Pages) TI1

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LP2954, LP2954A

SNVS096E – JUNE 1999 – REVISED JULY 2016

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*See External Capacitors

PD = (((VIN – VOUT) × IOUT) + (VIN × IG))

Figure 20. Basic 5-V 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) − TA(MAX)

where

•

TJ(MAX) is the maximum allowable junction temperature

•

TA(MAX) is the maximum ambient temperature

(8)

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

RθJA , can now be found:

RθJA = TR(MAX) / P(MAX)

(9)

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

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

sink can be calculated using the formula:

RθHA = RθJA − RθJC(bot) − RθCH

where

•

RθJC(bot) is the junction-to-case thermal resistance, which is specified as 0.9°C/W maximum for the LP2954IT

•

RθCH is the case-to-heat-sink thermal resistance, which is dependent on the interfacing material (if used). For

details and typical values in Table 2 and Table 3.

•

Rθ(H-A) is the heatsink-to-ambient thermal resistance. It is this specification (listed on the heat-sink

manufacturers data sheet) which defines the effectiveness of the heat sink. The heat sink selected must have

a thermal resistance which is equal to or lower than the value of RθHA calculated from the above listed

formula.

(10)

Table 2. Typical Values of Case-To-Heatsink Thermal

Resistance (RθCH) (Data from Aavid Engineering)

UNIT

(C°/W)

Silicone grease

Dry interface

1.3

Mica with grease

1.4

Table 3. Typical Values Of Case-To-Heatsink Thermal

Resistance (RθCH) (Data from Thermalloy)

UNIT

(C°/W)

Thermasil III

1.3

Thermasil II

1.5

Thermalfilm (0.002) with grease

2.2

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