AMC7580 datasheet(7/12 Pages) ETC1

7 / 12 page

AMC DOC. #: AMC7580_E (LF)

AMC7580

Feb 2005

7A LOW DROPOUT REGULATOR

Figure 1. Conventional Load Sensing

Figure 2. Remote Load Sensing

LOAD

ADJ

Power

VOUT

Sense

Control

Sense

VOUT

Power

ADJ

LOAD

(

∆I

OUT)(RP)

VOUT

Figure 2

VOUT

Figure 1

IOUT

Figure 3. Remote Sensing Improves Load Regulation

TIME

Stability

The Circuit design used in the AMC7580 series requires the use of an output capacitor as part of the device

frequency compensation. The addition of 150

µF Aluminum electrolytic or a 22µF 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 can respond

∆V = ∆t(∆I/C). Figure 4 illustrates these transient effects.

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