MIC2208 Datasheet, PDF(15/19 Page) Micrel Semiconductor – 3mmx3mm 1MHz 3A PWM Buck Regulator

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MIC2208 Datasheet, PDF (15/19 Pages) Micrel Semiconductor – 3mmx3mm 1MHz 3A PWM Buck Regulator

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Micrel, Inc.

MIC2208

Bill of Materials (cont.)

CRCW04023322F

CRCW04026192F

R2 CRCW04021003F

CRCW04022493F

R3 CRCW04024991F

R4 CRCW04024991F

R5 CRCW040210R0F

R6 CRCW04021002F

U1 MIC2208BML

33.2 kâ¦ 1% 0402 For 2.5VOUT

61.9 kâ¦ 1% 0402 For 1.8 VOUT

100 kâ¦ 1% 0402 For 1.5 VOUT

249 kâ¦ 1% 0402 For 1.2 VOUT

Open

For 1.0 VOUT

4.99Kâ¦ 1% 0402 resistor

90.9Kâ¦ 1% 0402 resistor

10â¦ 1% 0402 resistor

10Kâ¦ 1% 0402 resistor

1MHz 3A Buck Regulator

Vishay Dale

Micrel

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 7 demonstrates how a

transformer is used to inject a signal into the

feedback network.

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

âRâ Input

Feedback

+8V MIC922BC5

Network

Analyzer

âAâ Input

Output

Network Analyzer

Source

Figure 7. Transformer Injection

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

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 âsummedâ 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 analyzerâs 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 âRâ and output âAâ, 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

September 2005

M9999-092905

www.micrel.com

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