MIC4722 datasheet(13/20 Pages) MICREL

13 / 20 page

Micrel, Inc.

MIC4722

September 2007

M9999-090507-B

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 a transformer. Figure 7 demonstrates how a

transformer is used to inject a signal into the feedback

network.

Figure 7. 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. 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 to the tune

of several hundred dollars!). By using an op-amp, cost

and frequency limitations caused by an injection

transformer

are

completely

eliminated.

Figure

demonstrates using an op-amp in a summing amplifier

configuration for signal injection.

Network Analyzer

Source

+8V

Feedback

Output

Network

Analyzer

“A” Input

Network

Analyzer

“R” Input

MIC922BC5

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 50Ω 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 amplified by 2 by R3

and R4 (the 50Ω 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 supply’s 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 that the supply voltages need to be

below the maximum operating voltage of the op-amp.

Also, the maximum output voltage for driving 50Ω inputs

using the MIC922 is 3V. For measuring higher output

voltages, 1MΩ 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.

MIC4722 Datasheet(PDF) 13 Page - Micrel Semiconductor

MIC4722 Datasheet(HTML) 13 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL