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AS1370 − Application Information

Input Capacitor

An input capacitor at VIN is required for stability. It is

recommended that a 1.0μF capacitor be connected between

the AS1370 power supply input pin VIN and ground

(capacitance value may be increased without limit subject to

ESR limits). This capacitor must be located at a distance of not

more than 1cm from the VIN pin and returned to a clean analog

ground. Any good quality ceramic, tantalum, or film capacitor

may be used at the input.

Noise

The regulator output is a DC voltage with superimposed noise

on the output. The noise comes from three sources: the

reference, the error amplifier input stage and the output

voltage setting resistors. Noise is a random fluctuation: if noise

is not minimized in some applications, it will produce system

problems.

Load Transient Response

The series regulator is a negative feedback system: therefore

any change at the output will take a finite time to be corrected

by the error loop. This “propagation time” is related to the

bandwidth of the error loop. The initial response to an output

transient comes from the output capacitance; during this time,

ESR is the dominant mechanism causing voltage transients at

the output. More generally:

[V]

Thus an initial +50mA change of output current will produce a

-12mV transient when the ESR=240m

Ω. Remember to keep the

ESR within stability recommendations when reducing ESR by

adding multiple parallel output capacitors.

After the initial ESR transient, there follows a voltage drop

during the time that the LDO feedback loop takes to respond

to the output change. This drift is approximately linear in time

and sums with the ESR contribution to make a total transient

variation at the output of:

[V]

Where:

CLOAD = load capacitor [F];

T= propagation delay of the LDO [s].

(EQ14)

δV

TRANSIENT

δI

OUT

ESR

(EQ15)

δV

TRANSIENT

δI

OUT

ESR

LOAD

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