MIC2207_10 Datasheet, PDF (14/18 Pages) Micrel Semiconductor – 3mm x 3mm 2MHz 3A PWM 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
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 phase boost of 13.4 degrees
(47 degrees – 33.6 Degrees = 13.4 Degrees).
Output Impedance and Transient
response
Output impedance, simply stated, is the amount of
output voltage deviation vs. the load current deviation.
The lower the output impedance, the better.
Z _OUT
=
ΔV_OUT
ΔI_OUT
Output impedance for a buck regulator is the parallel
impedance of the output capacitor and the MOSFET and
inductor divided by the gain;
Z _TOTAL
=
R_DSON + DCR + X_L
GAIN
X _COUT
To measure output impedance vs. frequency, the load
current must be swept across the frequencies measured,
while the output voltage is monitored. Fig 9 shows a test
MIC2207
set-up to measure output impedance from 10Hz to 1MHz
using the MIC5190 high speed controller.
By setting up a network analyzer to sweep the feedback
current, while monitoring the output of the voltage
regulator and the voltage across the load resistance,
output impedance is easily obtainable. To keep the
current from being too high, a DC offset needs to be
applied to the network analyzer’s source signal. This can
be done with an external supply and 50Ω resistor. Make
sure that the currents are verified with an oscilloscope
first, to ensure the integrity of the signal measurement. It
is always a good idea to monitor the A and R
measurements with a scope while you are sweeping it.
To convert the network analyzer data from dBm to
something more useful (such as peak to peak voltage
and current in our case);
dBm
ΔV = ^{10 10 ×1mW × 50Ω × 2}
0.707
and peak to peak current;
dBm
ΔI = ^{10 10 × 1mW × 50Ω × 2}
0.707 × R_LOAD
The following graph shows output impedance vs
frequency at 2A load current sweeping the AC current
from 10Hz to 10MHz, at 1A peak to peak amplitude.
Output Impedance
vs. Frequency
1 _{V =1.8V}
OUT
L=1µH
C =4.7µF + 0.1µ
OUT
0.1
3.3VIN
0.01
5V
IN
0.00110 _{100 1k 10k 100k 1M}
FREQUENCY (Hz)
From this graph, you can see the effects of bandwidth
and output capacitance. For frequencies <200KHz, the
output impedance is dominated by the gain and
inductance. For frequencies >200KHz, the output
impedance is dominated by the capacitance. A good
approximation for transient response can be calculated
from determining the frequency of the load step in amps
per second;
f = ^A/sec
2π
April 2010
14
M9999-041910