TK71515AS 데이터 시트보기 (PDF) - Toko America Inc

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TK71515AS

LOW DROPOUT VOLTAGE REGULATOR

Toko America Inc  

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 PD /(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:

Tj = 0jA x PD + TA

For Toko ICs, the internal limit for junction temperature is

150 °C. If the ambient temperature (TA) is 25 °C, then:

150 °C = 0jA x PD + 25 °C

0jA = 125 °C/ PD

2)

PD1 is taken to be PD x (~ 0.8 - 0.9)

3)

Plot PD1 against 25 °C

4)

Connect PD1 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 PD against the point at which

the vertical line intersects the derating curve. This is taken

as the maximum power dissipation, DPD.

The maximum operating current is:

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

500

400

MOUNTED AS SHOWN

300

FREE AIR

200

100

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

simple way to determine PD 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.

0

0

50

100

150

TA (°C)

SOT23-3 POWER DISSIPATION CURVE

The range of usable currents can also be found from the

graph below.

(mW)

3

PD

DPD

6

4

5

25

50

75

150

TA (°C)

Procedure:

1)

Find PD

March 2001 TOKO, Inc.

Page 9

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