April 2004

11

M9999-042804

MIC5191

Micrel

Feedback Resistors

IR3716S

R1

COUT

R2

FB

GND

MIC5191

Figure 10. Adjustable Output

The feedback resistors adjust the output to the desired

voltage and can be calculated as follows:

VV

R1

R2

OUT

REF

=+









1

V

voltage (R1=0) is 0.5V. For output voltages less than 1V, use

the MIC5190.

The resistor tolerance adds error to the output voltage. These

errors are accumulative for both R1 and R2. For example, our

resistors selected have a

±1% tolerance. This will contribute

to a

±2% additional error on the output voltage.

The feedback resistors must also be small enough to allow

enough current to the feedback node. Large feedback resis-

tors will contribute to output voltage error.

VR1 I

V1k

1

A

V

mV

ERROR

FB

ERROR

ERROR

=×

=× µ

=

Ω

2

12

For our example application, this will cause an increase in

output voltage of 12mV. For the percentage increase,

V

V

V

V

12mV

1.5V

V

ERROR

ERROR

OUT

ERROR

ERROR

%

%

%. %

=×

=×

=

100

100

0 8

By reducing R1 to 100

Ω, the error contribution by the feed-

back resistors and feedback current is reduced to less than

0.1%. This is the reason R1 should not be greater than 100

Ω.

Applying the MIC5191

Linear Regulator

The primary purpose of the MIC5191 is as a linear regulator,

which enables an input supply voltage to drop down through

the resistance of the pass element to a regulated output

voltage.

Active Filter

Another application for the MIC5191 is as an active filter on

the output of a switching regulator. This improves the power

supply in several ways.

First, using the MIC5191 as a filter on the output can signifi-

cantly reduce high frequency noise. Switching power sup-

plies tends to create noise at the switching frequency in the

form of a triangular voltage ripple. High frequency noise is

also created by the high-speed switching transitions. A lot of

time, effort, and money are thrown into the design of switch-

ing regulators to minimize these effects as much as possible.

Figure 9 shows the MIC5191 as a post regulator.

TIME (1µs/div)

V

I

Figure 11. Ripple Reduction

Figure 11 shows the amount of ripple reduction for a 500 KHz

switching regulator. The fundamental switching frequency is

reduced from greater than 100mV to less than 10mV.

Figure 12. 10A Load Transient

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.

TIME (100

µs/div)