MIC5191BML(2004) 查看數據表（PDF） - Micrel
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MIC5191BML Datasheet PDF : 13 Pages
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MIC5191
This places a pole at 2.3kHz at 80dB and calculates as
follows.
FP
=
2π
×
1
3.42MΩ
×
20pF
FP = 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 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:
Internal
Error Amplifier
3.42MΩ
Driver
20pF
External
Comp
RCOMP
CCOMP
Figure 5. External Compensation
Placing an external capacitor (CCOMP) and resistor (RCOMP)
for the external pole-zero combination. Where the dominant
pole can be calculated as follows:
FP
=
2π
×
1
3.42MΩ
×
CCOMP
And the zero can be calculated as follows:
FZ
=
2π
1
× RCOMP
× CCOMP
This allows for high DC gain, and high bandwidth with the
output capacitor and the load providing the final pole.
Micrel
100
225
The Dominant Pole
1
80
Fp =
2 × 3.42 M × Ccomp
180
60
135
External Zero
1
Fz =
40
2 × Rcomp × Ccomp
90
R LOAD × COUT Pole
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 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.
100
225
80
180
60
Increasing COUT reduces
135
the load resistance and
output capacitor pole
40
allowing for an increase
90
in mid-band gain.
20
45
0
0
-20
0.01 0.1
-45
1
10 100 1000 10000 100000
Frequency (KHz)
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:
• VIN to VOUT differential
• Output Current
• Case Size/Thermal Characteristics
• Gate Capacitance (CISS<10nF)
• Gate to Source threshold
April 2004
8
M9999-042804
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