MIC5191

Micrel

April 20048

M9999-042804

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

follows.

F

M

pF

FkHz

P

P

=

××

=

1

23 4220

232

π

.

.

Ω

-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 resis-

tance 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

3.42M

Ω

20pF

Internal

External

Comp

Figure 5. External Compensation

Placing an external capacitor (C

for the external pole-zero combination. Where the dominant

pole can be calculated as follows:

F

MC

P

COMP

=

××

1

23 42

π

.Ω

And the zero can be calculated as follows:

F

RC

Z

COMP

COMP

=

××

1

2

π

This allows for high DC gain, and high bandwidth with the

output capacitor and the load providing the final pole.

Figure 6. External Compensation

Frequency Response

It is recommended that the gain bandwidth should be de-

signed to be less than 1 MHz. This is because most capaci-

tors lose capacitance at high frequency and becoming resis-

tive 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 increas-

ing of the compensation resistor, creating a higher mid-band

gain.

Figure 7. Increasing Output Capacitance

This will have the effect of both decreasing the voltage drop

as well as returning closer and faster to the regulated voltage

during the recovery time.

MOSFET Selection

The typical pass element for the MIC5191 is an N-Channel

MOSFET. There are multiple considerations when choosing

a MOSFET. These include:

• V

• Output Current

• Case Size/Thermal Characteristics

• Gate Capacitance (C

• Gate to Source threshold

-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

Pole

Ccomp

M

Fp

×

×

=

42

.

3

2

1

Ccomp

Rcomp

Fz

×

×

=

2

1

-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

Increasing C

the load resistance and

output capacitor pole

allowing for an increase

in mid-band gain.