CM1581
5A LOW DROPOUT VOLTAGE REGULATOR
2002/02/05
Preliminary Rev. 1
Champion Microelectronic Corporation
Page 7
Stability
The circuit design used in the CM1581 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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