March 2001 TOKO, Inc.

Page 9

TK715xxAS

DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)

determine the power dissipation for shutdown when

mounted, attach the device on the actual PCB and delib-

erately increase the output current (or raise the input

voltage) until the thermal protection circuit is activated.

Calculate the power dissipation of the device by subtract-

ing the output power from the input power. These mea-

surements should allow for the ambient temperature of the

PCB. The value obtained from P

derating factor. The PCB mounting pad should provide

maximum thermal conductivity in order to maintain low

device temperatures. As a general rule, the lower the

temperature, the better the reliability of the device. The

thermal resistance when mounted is expressed as follows:

T

For Toko ICs, the internal limit for junction temperature is

150 °C. If the ambient temperature (T

150 °C = 0

0

P

simple way to determine P

the output side is shorted. Input current gradually falls as

temperature rises. You should use the value when thermal

equilibrium is reached.

The range of usable currents can also be found from the

graph below.

Procedure:

1)

Find P

D

2)

P

3)

Plot P

4)

Connect P

°C with a straight line.

5)

In design, take a vertical line from the maximum

operating temperature (e.g., 75 °C) to the derating curve.

6)

Read off the value of P

the vertical line intersects the derating curve. This is taken

as the maximum power dissipation, D

The maximum operating current is:

I

SOT23-3 POWER DISSIPATION CURVE

PD

DPD

25

50

75

150

(mW)

TA (°C)

3

6

5

4

0

50

100

150

TA (°C)

0

300

500

100

200

400

MOUNTED AS SHOWN

FREE AIR