AMC DOC. #: AMC7580_E (LF)
AMC7580
Feb 2005
7A LOW DROPOUT REGULATOR
Figure 1. Conventional Load Sensing
Figure 2. Remote Load Sensing
R2
R1
RP
RP
LOAD
ADJ
Power
VOUT
Sense
Control
Control
Sense
VOUT
Power
ADJ
R2
R1
RP
RP
LOAD
(
∆I
OUT)(RP)
B
A
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.
Copyright
© 2002, ADD Microtech Corp.
7
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