MIC2207 Datasheet, PDF (16/21 Pages) Micrel Semiconductor – 3mmx3mm 2MHz 3A PWM Buck Regulator

Micrel
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.
Z _OUT
=
∆V_OUT
∆I_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. Fig 9 shows a test set-up to
measure output impedance from 10Hz to 1MHz
using the MIC5190 high speed controller.
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);
MIC2207
dBm
∆V = ^{10 10 ×1mW × 50Ω × 2}
0.707
and peak to peak current;
dBm
∆I = ^{10 10 × 1mW × 50Ω × 2}
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 _{V =1.8V}
OUT
L=1µH
C =4.7µF + 0.1µ
OUT
0.1
3.3VIN
0.01
5V
IN
0.00110 _{100 1k 10k 100k 1M}
FREQUENCY (Hz)
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;
f = ^A/sec
2π
April 2005
Figure 9. Output Impedance Measurement
16
M9999-040705
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