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AS1370 − Application Information

Total Accuracy

Away from dropout, total steady state accuracy is the sum of

setting accuracy, load regulation and line regulation. Generally:

Total Accuracy % = Setting Accuracy % + Load Regulation % + Line

Regulation %

Explanation of Dynamic Specifications

Power Supply Rejection Ratio (PSRR)

Known also as Ripple Rejection, this specification measures the

ability of the regulator to reject noise and ripple beyond DC.

PSRR depends on a summation of the individual rejections of

the error amplifier, reference and AC leakage through the series

pass transistor. The specification, in the form of a typical

attenuation plot with respect to frequency, shows up the gain

bandwidth compromises forced upon the designer in low load

current conditions. Generally:

PSSR =

[dB] using lower case

to indicate AC

values

Power supply rejection ratio is fixed by the internal design of

the regulator. Additional filtering must be provided externally.

Output Capacitor ESR

The series regulator is a negative feedback amplifier, and as

such is conditionally stable. The ESR of the output capacitor is

usually used to cancel one of the open loop poles of the error

amplifier in order to produce a single pole response; maximum

ESR should be less than 500m

Ω. Excessive ESR values may

actually cause instability by excessive changes to the closed

loop unity gain frequency crossover point. The range of ESR

values for stability is usually shown either by a plot of stable

ESR versus load current, or a limit statement in the datasheet.

Some ceramic capacitors exhibit large capacitance and ESR

variations in temperature. Z5U and Y5V capacitors may be

required to ensure stability at temperatures below TAMB = -10°C.

With X7R or X5R capacitors, a 1.0μF capacitor should be

sufficient at all operating temperatures.

Larger output capacitor values (>10μF) help to further reduce

noise and improve load transient-response, stability and

power-supply rejection.

(EQ12)

(EQ13)

20Log

δV

OUT

δV

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