TK11320B Ver la hoja de datos (PDF) - Toko America Inc

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TK11320B

VOLTAGE REGULATOR WITH ON/OFF SWITCH

Toko America Inc  

TK113xxB

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 shutdown. 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 is increased to 600 mW. For operation at ambient temperatures over 25 °C, the power

dissipation of the SOT-23L device should be derated at 4.8 mW/°C. The power dissipation of the SOT-89-5 package is

900 mW when mounted as recommended. Derate the power dissipation at 7.2 mW/°C for operation above 25 °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 X PD = 125 °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 currents usable can also be found from the graph below.

(mW)

3

PD

Dpd

6

4

5

Procedure:

25

50

75

150

T (°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.

May, 1997 TOKO, Inc.

Page 11

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