MIC5191_06 Datasheet, PDF (9/15 Pages) Micrel Semiconductor – Ultra High-Speed, High-Current Active Filter / LDO Controller

Micrel, Inc.
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.
Internal
Error Amplifier
Driver
20pF
External
Comp
Figure 3. Internal Compensation
This places a pole at 2.3 kHz at 80dB and calculates as
follows.
F_P
=
2π
1
× 3.42MΩ × 20pF
F_P = 2.32kHz
100
225
80
180
60
135
40
90
20
45
0
0
-20
0.01 0.1
-45
1
10 100 1000 10000 100000
Frequency (KHz)
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:
December 2006
Internal
Error Amplifier
Driver
MIC5191
20pF
External
Comp
R_COMP
C_COMP
Figure 5. External Compensation
Placing an external capacitor (C_COMP) and resistor
(R_COMP) for the external pole-zero combination. Where
the dominant pole can be calculated as follows:
F_P
=
2π
1
× 3.42MΩ × C_COMP
And the zero can be calculated as follows:
F_Z
=
1
2π × R_COMP
× C_COMP
This allows for high DC gain, and high bandwidth with
the output capacitor and the load providing the final pole.
100
225
The Dominant Pole
80
1
Fp
2 × 3.42M × Ccomp
180
60
135
External Zero
1
Fz
40
2 × Rcomp × Ccomp
90
R x C Pole
load
out
20
45
0
0
-20
0.01 0.1
-45
1
10 100 1000 10000 100000
Frequency (KHz)
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
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M9999-122206