MIC4782 Datasheet, PDF (15/23 Pages) Micrel Semiconductor – 1.8 MHz Dual 2A Integrated Switch Buck Regulator

Micrel, Inc.
Loop Stability and Bode Analysis
Bode analysis is an excellent way to measure small
signal stability and loop response in power supply
designs. Bode analysis monitors gain and phase of a
control loop. This is done by breaking the feedback loop
and injecting a signal into the feedback node and
comparing the injected signal to the output signal of the
control loop. This will require a network analyzer to
sweep the frequency and compare the injected signal to
the output signal. The most common method of injection
is the use of transformer. Figure 8 demonstrates how a
transformer is used to inject a signal into the feedback
network.
Figure 8. Transformer Injection
A 50Ω resistor allows impedance matching from the
network analyzer source. This method allows the DC
loop to maintain regulation and allow the network
analyzer to insert an AC signal on top of the DC voltage.
The network analyzer will then sweep the source while
monitoring A and R for an A/R measurement.
The following Bode analysis show the small signal loop
stability of the MIC4782, it utilizes type III compensation.
This is a dominant low frequency pole, followed by two
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).
MIC4782
Bode Plot
Vin=3.6V Vout=1.8V Iout=2A
60
210
50
175
40
140
30
105
20
70
10
35
0
0
-10
-35
-20
-70
-30
1.E+02
1.E+03 1.E+04 1.E+05
FREQUENCY (Hz)
-105
1.E+06
Typically for 3.6V_IN and 1.8V_OUT at 2A;
• Phase Margin = 77.8 Degrees
• GBW = 229KHz
Being that the MIC4782 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
Vin=3.6V Vout=1.8V Iout=0.1A
60
210
50
175
40
140
30
105
20
70
10
35
0
0
-10
-35
-20
-70
-30
1.E+02
1.E+03 1.E+04 1.E+05
FREQUENCY (Hz)
-105
1.E+06
3.6V_IN, 1.8V_OUT I_OUT = 0.1A;
• Phase Margin=89.9 Degrees
• GBW= 43.7kHz
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.
August 2009
15
M9999-081709-D