TK717 查看數據表（PDF） - Toko America Inc

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TK717

TK717xxS Series / LOW DROPOUT VOLTAGE REGULATOR

Toko America Inc  

TK717xxS

DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)

PACKAGE POWER DISSIPATION (PD)

This is the power dissipation level at which the thermal

sensor is activated. The IC contains an internal thermal

sensor which monitors the junction temperature. When the

junction temperature exceeds the monitor threshold of

150 °C, the IC is shut down. The junction temperature rises

as the difference between the input power (VIN x IIN) and the

output power (VOUT x IOUT) increases. The rate of tempera-

ture rise is greatly affected by the mounting pad configura-

tion on the PCB, the board material, and the ambient

temperature. When the IC mounting has good thermal

conductivity, the junction temperature will be low even if the

power dissipation is large. When mounted on the recom-

mended mounting pad, the power dissipation of the SOT23-

5 is increased to 500 mW. For operation at ambient

temperatures over 25 °C, the power dissipation of the

SOT23-5 device should be derated at 4.0 mW/°C. To

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 PD /(150 °C - TA) is the derating factor. The

PCB mounting pad should provide maximum thermal con-

ductivity 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:

The range of usable currents can also be found from the

graph below.

PD(mW)

3

PD

DPD

6

4

5

25

50

75

150

Procedure:

TA (°C)

1) Find 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)

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

0jA = 125 °C / PD (°C / mW)

PD is the value when the thermal protection circuit 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.

January 2002 TOKO, Inc.

Page 15

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