NC

GND

Vo

GND

NC

V+

NC

GND

Vo

GND

V+

LMT89

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SNIS176A – MARCH 2013 – REVISED JANUARY 2015

Layout Guidelines (continued)

The thermal resistance junction to ambient (RθJA) is the parameter used to calculate the rise of a device junction

temperature due to its power dissipation. Equation 13 is used to calculate the rise in the die temperature.

TJ = TA + RθJA [(V

+ I

Q) + (V

+ − V

O) IL]

where

•

IQ is the quiescent current and ILis the load current on the output. Because the junction temperature of the

LMT89 is the actual temperature being measured, take care to minimize the load current that the LMT89

device is required to drive.

(13)

Table 4 summarizes the rise in die temperature of the LMT89 device (without any loading), and the thermal

resistance for different conditions.

Table 4. Temperature Rise of LMT89 Due to Self-Heating and Thermal Resistance (RJΘA)

(1)

SC70-5

SC70-5

NO HEAT SINK

SMALL HEAT SINK

RθJA

TJ − TA

RθJA

TJ − TA

(°C/W)

(°C)

(°C/W)

(°C)

Still air

412

0.2

350

0.19

Moving air

312

0.17

266

0.15

(1)

See Layout Examples for PCB layout samples.

10.2 Layout Example

Figure 10. Layout Used for No Heat Sink Measurements

Figure 11. Layout Used for Measurements With Small Heat Sink

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