MIC2208_10 Datasheet, PDF (16/18 Pages) Micrel Semiconductor – 3mm x 3mm 1MHz 3A PWM Buck Regulator

Micrel, Inc.
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 gained up 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 supplies 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, that the supply voltages need to 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, a
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.
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.
Z _OUT
=
ΔV_OUT
ΔI_OUT
Output impedance for a buck regulator is the parallel
impedance of the output capacitor and the MOSFET and
inductor divided by the gain:
Z _TOTAL
=
R_DSON + DCR + X_L
GAIN
|| X_COUT
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.
MIC2208
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):
10 ^dBm × 1mW × 50Ω × 2
ΔV =
10
0.707
and peak to peak current:
10 ^dBm × 1mW × 50Ω × 2
ΔI =
10
0.707 × R_LOAD
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.
Output Impedance vs Frequency
1
3.3V_IN
0. 1
0. 01
5V _IN
V_OUT = 1.8V
L =1µH
C_OUT= 4.7µF+0.1µF
0. 00 1
10 10 0 1k
10 k 100 k 1M 10 M
Frequency (Hz)
May 2010
16
M9999-051410-D