Micrel, Inc.
MIC4720
Being that the MIC4720 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.
The following Bode analysis show the small signal loop
stability of the MIC4720, it utilizes type III compensation.
This is a dominant low frequency pole, followed by 2
zeros and finally the double pole of the inductor
capacitor filter, creating a final 20dB/decade roll off.
Bode analysis gives us a few important data points;
speed of response (Gain Bandwidth or GBW) and loop
stability. Loop speed or GBW determines the response
time to a load transient. Faster response times yield
smaller voltage deviations to load steps.
Instability in a control loop occurs when there is gain and
positive feedback. Phase margin is the measure of how
stable the given system is. It is measured by determining
how far the phase is from crossing zero when the gain is
equal to 1 (0dB).
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.
Typically for 3.3Vin and 1.8Vout at 2A;
•
•
Phase Margin=47 Degrees
GBW=156KHz
Gain and Phase
vs. Frequency
0
-1
25
20
15
10
5
L=1µH
COUT = 4.7µF
Gain will also increase with input voltage. The following
graph shows the increase in GBW for an increase in
supply voltage.
GAIN
-2
R1 = 10k
R2 = 12.4k
-3
C
FF = 82pF
-4
-5
PHASE
-6
-7
-8
-9
-10
0
100
1k
FREQUENCY (Hz)
1M
10k
100k
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.
5Vin, 1.8Vout at 2A load;
•
•
Phase Margin=43.1 Degrees
GBW= 218KHz
M9999-051707
May 2007
15
MICREL [ MICREL SEMICONDUCTOR ]
