Micrel, Inc.

MIC5191

December 2006

13

M9999-122206

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.

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.

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

Load

Switching

Power

Supply

Circuit Board

Load

Load

Long Traces

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

Load

Switching

Power

Supply

Circuit Board

Load

Load

MIC5191

MIC5191

MIC5191

MIC5191

Figure 15. Improved Distributed Supplies