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參數(shù)資料
| 型號(hào)： | MIC4720YML |
| 廠商： | MICREL INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 400 W Transient Voltage Suppressor, SOD123W (SOD2 FlatPower), Reel Pack, SMD |
| 中文描述： | 11 A SWITCHING REGULATOR, 2200 kHz SWITCHING FREQ-MAX, DSO12 |
| 封裝： | 3 X 3 MM, LEAD FREE, MLF-12 |
| 文件頁數(shù)： | 17/21頁 |
| 文件大小： | 914K |
| 代理商： | MIC4720YML |
Micrel, Inc.
MIC4720
May 2007
17
M9999-051707
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.
=
OUT
OUT
V
OUT
I
Z
Output impedance for a buck regulator is the parallel
impedance of the output capacitor and the MOSFET and
inductor divided by the gain;
+
=
COUT
L
DSON
TOTAL
X
GAIN
X
DCR
R
Z
+
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 50
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);
707
.
2
50
1mW
10
V
10
dBm
×
×
×
=
and peak to peak current;
LOAD
10
dBm
R
707
.
2
50
1mW
×
10
I
×
×
×
=
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.
0.001
0.01
0.1
1
O
FREQUENCY (Hz)
Output Impedance
vs. Frequency
100
1k
10k 100k
1M
V
OUT
=1.8V
L=1μH
C
OUT
=4.7μF + 0.1μ
5V
IN
3.3VIN
10
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;
π
2
A/sec
=
f
Then, determine the output impedance by looking at the
output impedance vs frequency graph. Then calculating
the voltage deviation times the load step;
OUT
OUT
OUT
V
Z
I
×
=
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.
COUT
L
DSON
TOTAL
X
GAIN
X
DCR
R
Z
↑
+
+
↓
=
↓
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