FC8731 datasheet(9/11 Pages) FS

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Input Capacitor

A minimum input capacitance of 2.2µF is required for FC8731. The capacitor value may be increased without

limit. Improper workbench set-ups may have adverse effects on the normal operation of the regulator. A case

in point is the instability that may result from long supply lead inductance coupling to the output through the

gate capacitance of the pass transistor. This will establish a pseudo LCR network, and is likely to happen under

high current conditions or near dropout. A 10µF tantalum input capacitor will dampen the parasitic LCR action

thanks to its high ESR. However, cautions should be exercised to avoid regulator short-circuit damage when

tantalum capacitors are used, for they are prone to fail in short-circuit operating conditions.

Power Dissipation and Thermal Shutdown

Thermal overload results from excessive power dissipation that causes the IC junction temperature to increase

beyond a safe operating level. The FC8731 relies on dedicated thermal shutdown circuitry to limit its total

power dissipation. An IC junction temperature TJ exceeding 165°C will trigger the thermal shutdown logic,

turning off the P-channel MOS pass transistor. The pass transistor turns on again after the junction cools off by

about 30°C. When continuous thermal overload conditions persist, this thermal shutdown action then results in

a pulsed waveform at the output of the regulator. The concept of thermal resistance θJA (°C/W) is often used

to describe an IC junction’s relative readiness in allowing its thermal energy to dissipate to its ambient air. An IC

junction with a low thermal resistance is preferred because it is relatively effective in dissipating its thermal

energy to its ambient, thus resulting in a relatively low and desirable junction temperature. The relationship

between θJA and TJ is as follows:

TJ = θJA x (PD) + TA

TA is the ambient temperature, and PD is the power generated by the IC and can be written as:

PD = IOUT (VIN – VOUT)

As the above equations show, it is desirable to work with Ics whose θJA values are small such that TJ does not

increase strongly with PD. To avoid thermally overloading the FC8731, refrain from exceeding the absolute

maximum junction temperature rating of 150°C under continuous operating conditions. Overstressing the

regulator with high loading currents and elevated input-to-output differential voltages can increase the IC die

temperature significantly.

Shutdown

The FC8731 enters sleep mode when the EN pin is low. When this occurs, the pass transistor, the error amplifier,

and the biasing circuits, including the bandgap reference, are turned off, thus reducing the supply current to

typically < 1uA. The low supply current makes the FC8731 best suited for battery-powered applications. The

maximum guaranteed voltage at the EN pin to enter sleep mode is 0.4V. A minimum guaranteed voltage of

1.2V at the EN pin will activate the FC8731. To constantly keep the regulator on, direct connection of the EN

pin to the VIN pin is allowed.

FC8731

2018. 05. 03

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Revision No : 1

FC8731 Datasheet(PDF) 9 Page - First Silicon Co., Ltd