MIC4723 Datasheet, PDF (15/19 Pages) Micrel Semiconductor – 3A 2MHz Integrated Switch Buck Regulator

Micrel, Inc.
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
0
1
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.
MIC4723
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 one 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 phase boost of 13.4 degrees
(47 degrees- 33.6 Degrees = 13.4 Degrees).
May 2007
15
M9999-052307-A