MIC5191_06 Datasheet, PDF (13/15 Pages) Micrel Semiconductor – Ultra High-Speed, High-Current Active Filter / LDO Controller

Micrel, Inc.
The transient response also contributes to the overall AC
output voltage deviation. Figure 12 shows a 1A to 10A
load transient. The top trace is the output of the
switching regulator (same circuit as Figure10). The
output voltage undershoots by 100mV. Just by their
topology, linear regulators have the ability to respond at
much higher speeds than a switching regulator. Linear
regulators do not have the limitation or restrictions of
switching regulators which must reduce their bandwidth
to less than their switching frequency.
Using the MIC5191 as a filter for a switching regulator
reduces output noise due to ripple and high frequency
switching noise. It also reduces undershoot (Figure 12)
and over-shoot (Figure 13) due to load transients with
decreased capacitance.
MIC5191
If a large circuit board has multiple small-geometry
ASICs, it will require the powering of multiple loads with
its one power source. Assuming that the ASICs are
dispersed throughout the board and that the core voltage
requires a regulated 1V, Figure 14 shows the long traces
from the power supply to the loads. Not only do we have
to contend with the tolerance of the supply (line
regulation, load regulation, output accuracy and
temperature tolerances), but the trace lengths create
additional issues with resistance and inductance. With
lower voltages these parasitic values can easily bump
the output voltage out of a usable tolerance.
Load
Circuit Board
Load
Load
Long Traces
Switching
Power
Supply
Load
Figure 13. Transient Response
Due to the high DC gain (80dB) of the MIC5191, it also
adds increased output accuracy and extremely high load
regulation.
Distributed Power Supply
As technology advances and processes move to smaller
and smaller geometries, voltage requirements go down
and current requirements go up. This creates unique
challenges when trying to supply power to multiple
devices on a board. When there is one load to power,
the difficulties are not quite as complex; trying to
distribute power to multiple loads from one supply is
much more problematic.
Figure 14. Board Layout
But by placing multiple, small MIC5191 circuits close to
each load, the parasitic trace elements caused by
distance to the power supply are almost completely
negated. By adjusting the switching supply voltage to
1.2V, for example, the MIC5191 will provide accurate 1V
output, efficiently and with very little noise.
Load
Circuit Board
MIC5191
MIC5191
Load
Load
MIC5191
Switching
Power
Supply
MIC5191
Load
Figure 15. Improved Distributed Supplies
December 2006
13
M9999-122206