MIC2208 MICREL [Micrel Semiconductor], MIC2208 Datasheet - Page 16

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

>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, that the supply voltages need
to below the maximum operating voltage of the op-
amp. Also, the maximum output voltage for driving
50 Ohm inputs using the MIC922 is 3V. For
measuring higher output voltages, a 1MOhm 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.
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.
Output impedance for a buck regulator is the parallel
impedance of the output capacitor and the MOSFET
and inductor divided by the gain;
To measure output impedance vs. frequency, the
load current must be 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.
September 2005
Z
Z
OUT
TOTAL
Figure 9. Output Impedance Measurement
=
=
∆
∆
V
I
R
OUT
OUT
DSON
GAIN
+
DCR
+
X
L
X
COUT
16
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 Ohm 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);
and peak to peak current;
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
<100KHz, the output impedance is dominated by the
gain and inductance. For frequencies >100KHz, the
0. 0 0 1
0 . 01
0 . 1
∆
∆
1
V
I
10
=
=
10
Output Impedance vs Frequency
10
10 0
dBm
10
dBm
. 0
10
707
×
×
1mW
1k
. 0
1mW
×
3.3V
707
R
LOAD
Frequency (Hz)
IN
×
×
10 k
50Ω
50Ω
V
L =1µH
C
OUT
OUT
×
×
10 0 k
2
5V
= 4.7µF+0.1µF
= 1.8V
2
IN
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M9999-092905
1M
10 M