Application Information

General Description

Referring to Fig.1 as shown in the Functional Block Diagram section, the FC8731 adopts the classical regulator

topology in which negative feedback control is used to perform the desired voltage regulating function. The

sub Vout-select form the feedback circuit which samples the output voltage for the error amplifier’s

non-inverting input. The inverting input is set to the bandgap reference voltage. Due to its high open-loop gain,

the error amplifier ensures that the sampled output feedback voltage at its non-inverting input is virtually equal

to the preset bandgap reference voltage. The error amplifier compares the voltage difference at its inputs and

produces an appropriate driving voltage to the P-channel MOS pass transistor, which controls the amount of

current reaching the output. If there are changes in the output voltage due to load changes, the feedback

resistors register these changes to the non-inverting input of the error amplifier. The error amplifier then adjusts its

driving voltage to maintain virtual short between its two input nodes under all loading conditions. The

regulation of the output voltage is achieved as a direct result of the error amplifier keeping its input voltages

equal. This negative feedback control topology is further augmented by the shutdown, the temperature and

current protection circuitry.

Selecting the Output Voltage

IC. FC8731 provides adjusted output voltage function al so via a resistor divider is connected to OUT, ADJ and

(FIG.2)

Output Capacitor

The FC8731 is specially designed for use with ceramic output capacitors of as low as 2.2μF to take advantage

of the savings in cost and space, as well as the superior filtering of high frequency noise. Capacitors of higher

value or other types may be used, but it is important to make sure its equivalent series resistance (ESR) be

restricted to less than 0.5Ω. The use of larger capacitors with smaller ESR values is desirable for applications

involving large and fast input or output transients, as well as situations where the application systems are not

physically located immediately adjacent to the battery power source. Typical ceramic capacitors suitable for

use with the FC8731 are X5R and X7R. The X5R and

the X7R capacitors are able to maintain their

capacitance values to within ±20% and ±10%, respectively, as the temperature increases.

No-Load Stability

The FC8731 is capable of stable operation during no-load conditions, a mandatory feature for some

applications such as CMOS RAM keep-alive operations.

2018. 05. 03

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Revision No : 1

FC8731