MIC2207 Datasheet, PDF (15/21 Pages) Micrel Semiconductor – 3mmx3mm 2MHz 3A PWM Buck Regulator

Micrel
MIC2207
Bode Plot
V =5V, V =1.8V, I =3A
60 ^IN
OUT
OUT
210
50
PHASE
175
40
140
30
105
20
70
10 _L=1µH
GAIN
0
C = 4.7µF
OUT
-10 ^{R1 = 10k}
R2 = 12.4k
-20 _{C = 82pF}
FF
-30
100 1k
10k
100k
FREQUENCY (Hz)
35
0
-35
-70
-105
1M
5Vin, 1.8Vout at 3A load;
• Phase Margin=43.1 Degrees
• GBW= 218KHz
Being that the MIC2207 is non-synchronous; the
regulator only has the ability to source current. This
means that the regulator has to rely on the load to
be able to sink current. This causes a non-linear
response at light loads. The following plot shows the
effects of the pole created by the nonlinearity of the
output drive during light load (discontinuous)
conditions.
Bode Plot
V =3.3V,V =1.8V,I =50mA
IN
OUT
OUT
60
210
50
175
40
PHASE
140
30
105
20
70
10 _L=1µH
0
C = 4.7µF
OUT
-10 ^{R1 = 10k}
R2 = 12.4k
-20 _{C = 82pF}
FF
-30
100 1k
GAIN
10k 100k
FREQUENCY (Hz)
35
0
-35
-70
-105
1M
3.3Vin, 1.8Vout Iout=50mA;
• Phase Margin=90.5 Degrees
• GBW= 64.4KHz
Feed Forward Capacitor
The feedback resistors are a gain reduction block in
the overall system response of the regulator. By
placing a capacitor from the output to the feedback
pin, high frequency signal can bypass the resistor
divider, causing a gain increase up to unity gain.
Gain and Phase
vs. Frequency
0 _L=1µH
25
-1 _{C = 4.7µF}
OUT
GAIN
-2 _{R1 = 10k}
20
-3 R2 = 12.4k
-4
C
FF
=
82pF
15
-5
-6
PHASE
10
-7
-8
5
-9
-10
100 1k
0
10k 100k 1M
FREQUENCY (Hz)
The graph above shows the effects on the gain and
phase of the system caused by feedback resistors
and a feedforward capacitor. The maximum amount
of phase boost achievable with a feedforward
capacitor is graphed below.
Max. Amount of Phase Boost
Obtainable using C vs. Output
FF
50
Voltage
45
40
35
30
25
20
15
10
5
V = 1V
REF
01
2
3
4
5
OUTPUT VOLTAGE (V)
By looking at the graph, phase margin can be
affected to a greater degree with higher output
voltages.
The next bode plot shows the phase margin of a
1.8V output at 3A without a feedforward capacitor.
Bode Plot
V =3.3V, V =1.8V, I =3A
IN
OUT
OUT
60
210
50
PHASE
175
40
140
30
105
20
70
10 _L=1µH
0
C = 4.7µF
OUT
-10 ^{R1 = 10k}
GAIN
R2 = 12.4k
-20 _{C = 0pF}
FF
-30
100 1k 10k
100k
FREQUENCY (Hz)
35
0
-35
-70
-105
1M
As you can see the typical phase margin, using the
same resistor values as before without a
feedforward capacitor results in 33.6 degrees of
phase margin. Our prior measurement with a
feedforward capacitor yielded a phase margin of 47
degrees. The feedforward capacitor has given us a
April 2005
15
M9999-040705
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