MIC5190 Micrel Semiconductor, MIC5190 Datasheet - Page 8

• Manufacturer Part Number: MIC5190
• Description: Ultra High-Speed High-Current Active Filter/LDO Controller
• Manufacturer: Micrel Semiconductor

MIC5190
This places a pole at 2.3 kHz at 80dB and calculates as
follows.
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. External compensation is required. By provid-
ing 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 placing an external capacitor (C
resistor (R
Where the dominant pole can be calculated as follows:
And the zero can be calculated as follows:
This allows for high DC gain, and high bandwidth with the
output capacitor and the load providing the final pole.
December 2005
There is single pole roll off. For most applications, an output
F
F
F
F
P
P
P
Z
2 32
2
2
2
COMP
Figure 5. External Compensation
Figure 4. Internal Compensation
100
-20
80
60
40
20
0
0.01
kHz
R
3 42
3 42
) for the external pole-zero combination.
COMP
Error Amplifier
Frequency Response
0.1
1
M
M
1
1
External
20pF
C
1
COMP
20
3.42M
C
C
Frequency (KHz)
R
Internal
COMP
COMP
pF
10
COMP
100
Comp
Driver
1000
10000 100000
225
180
135
90
45
0
-45
COMP
) and
8
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.
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 MIC5190 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
Figure 7. Increasing Output Capacitance
IN
to V
Figure 6. External Compensation
100
-20
80
60
40
20
100
OUT
-20
0
80
60
40
20
0.01
0
0.01
Frequency Response
differential
0.1
0.1
Fp =
Fz =
The Dominant Pole
2
1
1
2
. 3
External Zero
Frequency (KHz)
ISS
42
Frequency (KHz)
10
R comp
M
10
1
<10nF)
1
C comp
100
100
C comp
Increasing C
the load resistance and
allowing for an increase
output capacitor pole
in mid-band gain.
R
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1000
LOAD
1000
OUT
C
10000 100000
OUT
10000 100000
reduces
Pole
225
180
135
90
45
0
-45
225
180
135
90
45
0
-45
M9999-120105
Micrel