MIC5190
Micrel
This places a pole at 2.3 kHz at 80dB and calculates as
follows.
100
80
60
40
20
0
225
180
135
90
1
The Dominant Pole
F =
P
1
2π × 3.42MΩ × 20pF
Fp =
2
× 3.42M × C
comp
F = 2.32kHz
P
External Zero
1
Fz =
2
× R
comp
C
×
comp
R LOAD × COUT Pole
100
80
225
180
135
45
0
60
-20
-45
40
20
0
90
45
0
0.01
0.1
1
10
100
1000
10000 100000
Frequency (KHz)
Figure 6. External Compensation
Frequency Response
-20
-45
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.
0.01
0.1
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. 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
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
capacitordecreasesinfrequency.Thisallowsfortheincreas-
ing of the compensation resistor, creating a higher mid-band
gain.
100
80
60
40
20
0
225
180
135
90
demonstrates placing an external capacitor (C
) and
COMP
resistor (R
) for the external pole-zero combination.
COMP
Where the dominant pole can be calculated as follows:
Increasing C
reduces
OUT
the load resistance and
output capacitor pole
allowing for an increase
in mid-band gain.
Internal
45
Error Amplifier
3.42MΩ
Driver
0
20pF
-20
-45
0.01
0.1
1
10
100
1000
10000 100000
Frequency (KHz)
External
Comp
RCOMP
CCOMP
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.
Figure 5. External Compensation
MOSFET Selection
The typical pass element for the MIC5190 is an N-Channel
MOSFET. There are multiple considerations when choosing
a MOSFET. These include:
1
F =
P
2π × 3.42MΩ × CCOMP
• V to V
differential
OUT
IN
And the zero can be calculated as follows:
• Output current
1
• Case size/thermal characteristics
FZ =
2π ×RCOMP × CCOMP
• Gate capacitance (C <10nF)
ISS
This allows for high DC gain, and high bandwidth with the
• Gate to source threshold
output capacitor and the load providing the final pole.
December 2005
8
M9999-120105
MICREL [ MICREL SEMICONDUCTOR ]
