MIC4721 Datasheet, PDF (15/19 Pages) Micrel Semiconductor – 1.5A 2MHz Integrated Switch Buck Regulator

Micrel, Inc.
As one can see the typical phase margin, using the
same resistor values as before without a feedforward
capacitor results in 33.6 degrees of phase margin. Our
prior measurement with a feedforward capacitor yielded
a phase margin of 47 degrees. The feedforward
capacitor has given us a 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 +
GAIN
X_L
||
X _COUT
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.
MIC4721
10 ^dBm × 1mW × 50Ω × 2
∆V =
10
.707
and peak to peak current:
∆V =
10 ^dBm × 1mW × 50Ω × 2
10
.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.
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):
May 2007
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
Then, determine the output impedance by looking at the
output impedance vs frequency graph. Next, calculate
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 R_DSON 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 + X _L
↑ GAIN
||
X _COUT
To properly measure ripple on either input or output of a
switching regulator, a proper ring in tip measurement is
required. Standard oscilloscope probes come with a
grounding clip, or a long wire with an alligator clip.
15
M9999-052907-A