MIC2208 MICREL [Micrel Semiconductor], MIC2208 Datasheet - Page 15

• Manufacturer Part Number: MIC2208
• Description: 3mmx3mm 1MHz 3A PWM Buck Regulator
• Manufacturer: MICREL [Micrel Semiconductor]

Bill of Materials (cont.)
R2
R3
R4
R5
R6
U1
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.
transformer is used to inject a signal into the
feedback network.
A 50 ohm 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. 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
September 2005
CRCW04023322F
CRCW04026192F
CRCW04021003F
CRCW04022493F
CRCW04024991F
CRCW04024991F
CRCW040210R0F
CRCW04021002F
MIC2208BML
Figure 7. Transformer Injection
Figure
7
demonstrates
33.2 k Ω 1% 0402 For 2.5V
61.9 k Ω 1% 0402 For 1.8 V
100 k Ω 1% 0402 For 1.5 V
249 k Ω 1% 0402 For 1.2 V
Open
4.99K Ω 1% 0402 resistor
90.9K Ω 1% 0402 resistor
10 Ω 1% 0402 resistor
10K Ω 1% 0402 resistor
1MHz 3A Buck Regulator
how
a
For 1.0 V
15
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.
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
OUT
Feedback
OUT
OUT
Network
Analyzer
“R” Input
OUT
OUT
Figure 8. Op Amp Injection
R3
1k
+8V
MIC922BC5
R4
1k
50
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
Micrel
Network Analyzer
Source
R1
www.micrel.com
1k
M9999-092905
Network
Analyzer
“A” Input
Output
1
1
1
1
1
1