MIC2207 Datasheet, PDF (14/21 Pages) Micrel Semiconductor – 3mmx3mm 2MHz 3A PWM Buck Regulator

Micrel
MIC2207
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. While this is the most common
method for measuring the gain and phase of a
power supply, it does have significant limitations.
First, to measure low frequency gain and phase, the
transformer needs to be high in inductance. This
makes frequencies <100Hz require an extremely
large and expensive transformer. Conversely, it must
be able to inject high frequencies. Transformers with
these wide frequency ranges generally need to be
custom made and are extremely expensive (usually
in the tune of several hundred dollars!). By using an
op-amp, cost and frequency limitations used by an
injection transformer are completely eliminated.
Figure 8 demonstrates using an op-amp in a
summing amplifier configuration for signal injection.
Network
Analyzer
“R” Input
Feedback
+8V _MIC922BC5
R1
1k
Network
Analyzer
“A” Input
Output
R3
1k
R4
1k
Network Analyzer
Source
50
Figure 8. Op Amp Injection
R1 and R2 reduce the DC voltage from the output to
the non-inverting input by half. The network analyzer
is generally a 50 Ohm source. R1 and R2 also divide
the AC signal sourced by the network analyzer by
half. These two signals are “summed” together at
half of their original input. The output is then gained
up by 2 by R3 and R4 (the 50 Ohm is to balance the
network analyzer’s source impedance) and sent to
the feedback signal. This essentially breaks the loop
and injects the AC signal on top of the DC output
voltage and sends it to the feedback. By monitoring
the feedback “R” and output “A”, gain and phase are
measured. This method has no minimum frequency.
Ensure that the bandwidth of the op-amp being used
is much greater than the expected bandwidth of the
power supplies control loop. An op-amp with
>100MHz bandwidth is more than sufficient for most
power supplies (which includes both linear and
switching) and are more common and significantly
cheaper than the injection transformers previously
mentioned. The one disadvantage to using the op-
amp injection method, is the supply voltages need to
below the maximum operating voltage of the op-
amp. Also, the maximum output voltage for driving
50 Ohm inputs using the MIC922 is 3V. For
measuring higher output voltages, a 1MOhm input
impedance is required for the A and R channels.
Remember to always measure the output voltage
with an oscilloscope to ensure the measurement is
working properly. You should see a single sweeping
sinusoidal waveform without distortion on the output.
If there is distortion of the sinusoid, reduce the
amplitude of the source signal. You could be
overdriving the feedback causing a large signal
response.
The following Bode analysis show the small signal
loop stability of the MIC2207. The MIC2207 utilizes
a type III compensation. This is a dominant low
frequency pole, followed by 2 zero’s 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).
Bode Plot
V =3.3V, V =1.8V, I =3A
60 ^IN
OUT
OUT ₂₁₀
50
PHASE
175
40
140
30
105
20
70
10 _L=1µH
0
C = 4.7µF
OUT
GAIN
-10 ^{R1 = 10k}
R2 = 12.4k
-20 _{C = 82pF}
FF
-30
100 1k 10k
100k
FREQUENCY (Hz)
35
0
-35
-70
-105
1M
Typically for 3.3Vin and 1.8Vout at 3A;
• Phase Margin=47 Degrees
• GBW=156KHz
Gain will also increase with input voltage. The
following graph shows the increase in GBW for an
increase in supply voltage.
April 2005
14
M9999-040705
www.micrel.com