TK71541 datasheet(9/13 Pages) TOKO

9 / 13 page

January 1999 TOKO, Inc.

Page 9

TK715xx

DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)

determine the power dissipation for shutdown when

mounted, attach the device on the actual PCB and

deliberately increase the output current (or raise the input

voltage) until the thermal protection circuit is activated.

Calculate the power dissipation of the device by subtracting

the output power from the input power. These

measurements should allow for the ambient temperature

of the PCB. The value obtained from P

D /(150 °C - TA) is the

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:

j = 0jA x PD + TA

For Toko ICs, the internal limit for junction temperature is

150

°C. If the ambient temperature (T

A) is 25 °C, then:

150

°C = 0

jA x PD + 25 °C

jA = 125 °C/ PD

D is the value when the thermal sensor is activated. A

simple way to determine P

D is to calculate VIN x IIN when

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

2) P

D1 is taken to be PD x (~ 0.8 - 0.9)

3) Plot P

D1 against 25 °C

4) Connect P

D1 to the point corresponding to the 150 °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

D against the point at which the

vertical line intersects the derating curve. This is taken

as the maximum power dissipation, D

PD.

The maximum operating current is:

OUT = (DPD / (VIN(MAX) - VOUT)

SOT-23-3 POWER DISSIPATION CURVE

DPD

150

(mW)

TA (°C)

100

150

TA (°C)

300

500

100

200

400

MOUNTED AS

SHOWN

FREE AIR

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