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AS1358 / AS1359

Datasheet - Application In form atio n

9.6.2 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. 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 with temperature. Z5U and Y5V capacitors may be required to ensure

Larger output capacitor values (2.2µF max) help to reduce noise and improve load transient-response, stability and power-supply rejection.

9.6.3 Input Capacitor

be used at the input.

9.6.4 Noise

The regulator output is a DC voltage with noise superimposed 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 and if not minimized in some applications, will

produce system problems. The AS1358/9 architecture provides enhance noise reduction when an external 10nF capacitor is connected between

Bypass and Output pins, and 1µF connected as the output capacitor.

The leakage current going into the BYPASS pin should be less than 10nA. Increasing the capacitance slightly decreases the output noise.

Values above 0.1µF and below 0.001µF are not recommended.

9.6.5 Transient Response

The series regulator is a negative feedback system, and 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, and during this time, ESR is the dominant mechanism causing voltage transients at the output. More generally:

Units are Volts, Amps, Ohms.

(EQ 14)

Thus an initial +50mA change of output current will produce a -12mV transient when the ESR=240m

. 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 droop 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:

Units are Volts, Seconds, Farads, Ohms.

(EQ 15)

Where:

T = Propagation delay of the LDO

This shows why it is convenient to increase the output capacitor value for a better support for fast load changes. Of course the formula holds for

t < “propagation time”, so that a faster LDO needs a smaller cap at the load to achieve a similar transient response. For instance 50mA load

current step produces 50mV output drop if the LDO response is 1usec and the load cap is 1µF.

There is also a steady state error caused by the finite output impedance of the regulator. This is derived from the load regulation specification

discussed above.

9.6.6 Turn On Time

This specification defines the time taken for the LDO to awake from shutdown. The time is measured from the release of the enable pin to the

Shutdown reduces the quiescent current to very low, mostly leakage values (<1µA).

9.6.7 Thermal Protection

To prevent operation under extreme fault conditions, such as a permanent short circuit at the output, thermal protection is built into the device.

Die temperature is measured, and when a 160ºC threshold is reached, the device enters shutdown. When the die cools sufficiently, the device

will restart (assuming input voltage exists and the device is enabled). Hysteresis of 15ºC prevents low frequency oscillation between start-up and

shutdown around the temperature threshold.

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TRANSIENT

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TRANSIENT

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