TK11120M Просмотр технического описания (PDF) - Toko America Inc

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TK11120M

VOLTAGE REGULATOR WITH ON/OFF SWITCH

Toko America Inc  

TK111xxM

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

temperature rise is greatly affected by the mounting pad

configuration 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 great. When mounted on

the recommended mounting pad, the power dissipation of

the SOT-23L-6 is increased to 600 mW. For operation at

ambient temperatures over 25 °C, the power dissipation of

the SOT-23L-6 device should be derated at 4.8 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 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

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.

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

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.

1.0

0.8

MOUNTED AS

SHOWN

0.6

FREE AIR

0.4

0.2

0

0

50

100

150

TA (°C)

SOT-23L-6 POWER DISSIPATION CURVE

March 1999 TOKO, Inc.

Page 11

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