MIC4721 datasheet(15/19 Pages) MICREL

15 / 19 page

Micrel, Inc.

MIC4721

May 2007

M9999-052907-A

As one can see the typical phase margin, using the

same resistor values as before without a feedforward

capacitor results in 33.6 degrees of phase margin. Our

prior measurement with a feedforward capacitor yielded

a phase margin of 47 degrees. The feedforward

capacitor has given us a phase boost of 13.4 degrees

(47 degrees – 33.6 Degrees = 13.4 Degrees).

Output Impedance and Transient Response

Output impedance, simply stated, is the amount of

output voltage deviation vs. the load current deviation.

The lower the output impedance, the better.

OUT

∆

Output impedance for a buck regulator is the parallel

impedance of the output capacitor and the MOSFET and

inductor divided by the gain:

COUT

DSON

TOTAL

GAIN

DCR

To measure output impedance vs. frequency, the load

current must be swept across the frequencies measured,

while the output voltage is monitored. Figure 9 shows a

test set-up to measure output impedance from 10Hz to

1MHz using the MIC5190 high speed controller.

Figure 9. Output Impedance Measurement

By setting up a network analyzer to sweep the feedback

current, while monitoring the output of the voltage

regulator and the voltage across the load resistance,

output impedance is easily obtainable. To keep the

current from being too high, a DC offset needs to be

applied to the network analyzer’s source signal. This can

be done with an external supply and 50

Ω resistor. Make

sure that the currents are verified with an oscilloscope

first, to ensure the integrity of the signal measurement. It

is always a good idea to monitor the A and R

measurements with a scope while you are sweeping it.

To convert the network analyzer data from dBm to

something more useful (such as peak to peak voltage

and current in our case):

707

Ω

∆

dBm

and peak to peak current:

LOAD

dBm

Ω

∆

707

The following graph shows output impedance vs

frequency at 2A load current sweeping the AC current

from 10Hz to 10MHz, at 1A peak-to-peak amplitude.

From this graph, you can see the effects of bandwidth

and output capacitance. For frequencies <200KHz, the

output impedance is dominated by the gain and

inductance.

For

frequencies

>200KHz,

the

output

impedance is dominated by the capacitance. A good

approximation for transient response can be calculated

from determining the frequency of the load step in amps

per second:

sec

Then, determine the output impedance by looking at the

output impedance vs frequency graph. Next, calculate

the voltage deviation times the load step;

∆V

OUT = ∆IOUT ×ZOUT

The output impedance graph shows the relationship

between supply voltage and output impedance. This is

caused by the lower RDSON of the high side MOSFET

and the increase in gain with increased supply voltages.

This explains why higher supply voltages have better

transient response.

COUT

DSON

TOTAL

GAIN

DCR

↑

↓

To properly measure ripple on either input or output of a

switching regulator, a proper ring in tip measurement is

required. Standard oscilloscope probes come with a

grounding clip, or a long wire with an alligator clip.

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