CS5231−3

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7

“Oscillatory Performance” Mode Lasts Longer

Greater

STABILITY CONSIDERATIONS

The output capacitor helps determine three main

characteristics of a linear regulator: startup, transient

response and stability.

Startup is affected because the output capacitor must be

present, and the output capacitor will be charged through the

PFET. The PFET will initially provide current to the load

through its body diode. The diode will act as a voltage

follower until sufficient voltage is present to turn the FET

on. Since most commercial power supplies have a fairly low

ramp rate, charging through the body diode should

effectively limit in−rush current to the capacitor.

During

normal

operation,

transient

load

current

requirements will be satisfied from the charge stored in the

output capacitor until either the linear regulator or the

auxiliary supply can respond. Larger values of capacitance

will improve transient response, but will also cost more. A

linear regulator will respond within microseconds, where an

external power supply may take milliseconds to react. The

output capacitance will provide the difference in current

until this occurs. The result will be an instantaneous voltage

change at the output. This change is the product of the

current change and the capacitor ESR:

DVOUT + DILOAD

ESR

This limitation directly affects load regulation. Capacitor

ESR must be minimized if output voltage must be

maintained within tight tolerances. In such a case, it is often

advisable to use a parallel network of different types of

capacitors. For example, electrolytic capacitors provide

high charge storage capacity in a small size, while tantalum

capacitors have low ESR. The parallel combination will

result in a high capacity, low ESR network. It is also

important to physically locate the capacitance network close

to the load, and to connect the network to the load with wide

PC board traces to minimize the metal resistance.