Micrel, Inc.

MIC5191

December 2006

9

M9999-122206

Compensation

The MIC5191 allows the flexibility of externally

controlling the gain and bandwidth. This allows the

MIC5191 design to be tailored to each individual design.

In designing the MIC5191, it is important to maintain

adequate phase margin. This is generally achieved by

having the gain cross the 0dB point with a single pole

20dB/decad roll-off. The compensation pin is configured

as Figure 3 demonstrates.

Error Amplifier

Driver

20pF

Internal

External

Comp

Figure 3. Internal Compensation

This places a pole at 2.3 kHz at 80dB and calculates as

follows.

20pF

3.42MΩ

2

1

×

×

=

π

-20

0

20

40

60

80

100

0.01

0.1

1

10

100

1000

10000 100000

Frequency (KHz)

-45

0

45

90

135

180

225

Figure 4. Internal Compensation

Frequency Response

There is single pole roll off. For most applications, an

output capacitor is required. The output capacitor and

load resistance create another pole. This causes a two-

pole system and can potentially cause design instability

with inadequate phase margin. What should we do?

Answer

: we compensate it externally. By providing a

dominant pole and zero–allowing the output capacitor

and load to provide the final pole–a net single pole roll

off is created, with the zero canceling the dominant pole.

Figure 5 demonstrates:

Error Amplifier

Driver

20pF

Internal

External

Comp

Figure 5. External Compensation

the dominant pole can be calculated as follows:

COMP

P

C

3.42MΩ

2

1

F

×

×

=

π

And the zero can be calculated as follows:

COMP

COMP

Z

C

R

2

1

F

×

×

=

π

This allows for high DC gain, and high bandwidth with

the output capacitor and the load providing the final pole.

-20

0

20

40

60

80

100

0.01

0.1

1

10

100

1000

10000 100000

Frequency (KHz)

-45

0

45

90

135

180

225

The Dominant Pole

External Zero

R

Ccomp

M

Fp

×

×42

.

3

2

1

Ccomp

Rcomp

Fz

×

×

2

1

Figure 6. External Compensation

Frequency Response

It is recommended that the gain bandwidth should be

designed to be less than 1 MHz. This is because most

capacitors lose capacitance at high frequency and

becoming resistive or inductive. This can be difficult to

compensate for and can create high frequency ringing or

worse, oscillations.

By increasing the amount of output capacitance,

transient response can be improved in multiple ways.

First, the rate of voltage drop vs. time is decreased.

Also, by increasing the output capacitor, the pole formed

by the load and the output capacitor decreases in

frequency. This allows for the increasing of the