MIC4722 datasheet(16/20 Pages) MICREL

16 / 20 page

Micrel, Inc.

MIC4722

September 2007

M9999-090507-B

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 50Ω 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

50Ω

1mW

dBm

∆

and peak to peak current;

LOAD

dBm

707

50Ω

1mW

∆

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.

0.001

0.01

0.1

FREQUENCY (Hz)

Output Impedance

vs. Frequency

100

10k 100k 1M

OUT

=1.8V

L=1µH

OUT

=4.7µF + 0.1µ

3.3VIN

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;

A/sec

Then, determine the output impedance by looking at the

output impedance vs frequency graph. Next calculate

the voltage deviation times the load step;

OUT

∆

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

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