MIC4724 Datasheet, PDF (16/19 Pages) Micrel Semiconductor – 3A 2MHz Integrated Switch Buck Regulator with 6Vmax Input

Micrel, Inc.
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. 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);
dBm
∆V = ^{10 10 ×1mW × 50Ω × 2}
0.707
and peak-to-peak current;
MIC4724
dBm
∆I = ^{10 10 × 1mW × 50Ω × 2}
0.707 × R_LOAD
The following graph shows output impedance vs
frequency at 3A 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, one 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
Then, determine the output impedance by looking at the
output impedance vs frequency graph. Then calculating
the voltage deviation times the load step;
∆V_OUT = ∆I_OUT × Z_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.
↓Z_TOTAL =
↓
R_DSON + DCR
↑ GAIN
+
X_L
X _COUT
June 2008
16
M9999-062408-A