CM1582S datasheet(7/11 Pages) ETC2

7 / 11 page

CM1582

3A LOW DROPOUT VOLTAGE REGULATOR

2002/02/05

Preliminary Rev. 1

Champion Microelectronic Corporation

Page 7

Stability

The circuit design used in the CM1582 series requires the use of an output capacitor as part of the device frequency

compensation. The addition of 150uF Aluminum electrolytic or a 22uF solid tantalum on the output will ensure stability for all

operating conditions. In order to meet the transient performance of the processor larger value capacitors are needed. To limit the

high frequency noise generated by the processor, high quality bypass capacitors must be used. In order to limit parasitic

inductance (ESL) and resistance (ESR) in capacitors to acceptable limits, multiple small ceramic capacitor in addition to high

quality solid tantalum capacitors are required.

When the adjustment terminal is bypassed to improve the ripple rejection, the requirement for an output capacitor increases. To

further improve stability and transient response of these devices larger values of output capacitor can be used. The modern

processors generate large high frequency current transients.

The load current step contains higher order frequency components than the output-coupling network must handle until the

regulator throttles to the load current level. Because they contain parasitic resistance and inductance, capacitors are not ideal

elements. These parasitic elements dominate the change in output voltage at the beginning of a transient load step change. The

ESR of the output capacitors produces and instantaneous step in output voltage at the beginning of a transient load step change.

The ESR of the output capacitors produces an instantaneous step in out voltage ∆V = ∆I(ESR). The ESL of the output capacitors

produces a droop proportional to the rate of change of the output current ∆V = L(∆I/∆t). The output capacitance produces a

change in output voltage proportional to the time until the regulator an respond ∆V = ∆t(∆I/C). Figure 4 illustrates these transient

effects.

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