January 1999 TOKO, Inc.

Page 7

TK70403

TIME (µs)

0

50

CL = 0.47 µF

CL = 2.2 µF

CL = 4.7 µF

CL = 0.68 µF

REDUCTION OF OUTPUT NOISE

Although the architecture of the Toko regulators is designed

to minimize semiconductor noise, further reduction can be

achieved by increasing the size of the output capacitor. A

more effective solution would be to add a capacitor to the

noise bypass terminal. The value of the capacitor should

be at least 0.1

µF or higher (higher values provide greater

noise reduction). Although stable operation is possible

without the noise bypass capacitor, this terminal has a high

impedance and care should be taken to avoid a large

circuit area on the printed circuit board when the capacitor

is not used. Please note that several parameters are

affected by the value of the capacitors and bench testing

is recommended when deviating from standard values.

INPUT AND OUTPUT CAPACITORS

Toko regulators require an output capacitor in order to

maintain regulator loop stability. The capacitor value should

be at least 0.68

µF over actual ambient operating

temperature.

PACKAGE POWER DISSIPATION (P

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 (V

and the output power (V

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

DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.)

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-26 is increased to 350 mW. For operation at

ambient temperatures over 25

°C, the power dissipation of

the SOT-26 device should be derated at 2.8 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 P

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:

T

For Toko ICs, the internal limit for junction temperature is

150

°C. If the ambient temperature (T

150

°C = 0

0

P

simple way to determine P

the output side is shorted. Input current gradually falls as

temperature rises. You should use the value when thermal

equilibrium is reached.