字母英文DESIGN IDEAS L Eliminate EMI Worries with 2A,
15mm × 9mm × 2.82mm
µModule Step-Down Regulator
by David Ng Introduction
“We failed EMI.” Tho three dreaded
words strike fear into the hearts and
minds of electronics design engineers.
There are four words that are even
wor: “We failed EMI again.” The
psyche of many a asoned engineer
is scarred with dark memories of long
days and nights in an EMI lab, strug-
gling with aluminum foil, copper tape,
clamp-on filter beads and finger cuts to
fix a design that just won’t keep quiet. A
big part of the problem is the necessary
profusion of switching power supplies,
which contribute significantly to the radiated system EMI.
The LTM8032 is a DC/DC switching step-down µModule regulator built specifically for low EMI. It is rated for up to 36V IN, 10V OUT at 2A, and features adjustable frequency, synchroniza-tion, a power good status flag and soft-start. It is small, measuring only 15mm × 9mm × 2.82mm, integrating the inductor, power stage and control-ler in a ROHS e3-compliant molded LGA package.
10V/2A Supply Is
EN55022 and CSIPR 22 Class B Compliant浅唱歌词
Like most other µModule regula-tors, the LTM8032 is easy to u. As shown in Figure 1, all that is needed for a complete power design are the resistors to t the output voltage and operating frequency, and the input and output caps.
The LTM8032 is test-proven EN55022 and CSIPR 22 class B complaint, tested in an NRTL 5-meter chamber, t up as shown in the photo given in Figure 2. The LTM8032 is mounted on a circuit board with no bulk capacitance installed. The input and output capacitance are the mini-mum ceramic values specified in the data sheet for proper operation.
The asmbled unit is placed
atop an all-wood table. The all-wood
construction ensures that the test
tup does not shield or shadow noi
emanating from the device under test
(DUT). The power source, a linear lab
grade power supply, is on the floor. The
load for the LTM8032, with its heat
sink, is also on the table top.
Before measuring the emissions
from the LTM8032, a baline mea-
surement is taken to establish the The LTM8032 is a
DC/DC switching step-down
µModule regulator built
for low EMI. It is rated for
up to 36V IN, 10V OUT at 2A,
integrating the inductor,
power stage and controller
in a ROHS e3-compliant
爱心树故事molded LGA package.
OUT
V
Figure 1. Just two resistors, input and output caps are needed
to complete a power supply design with the LTM8032.
Figure 2. For EMI testing, the DUT is mounted on a circuit board
and placed on a wooden table. The power source is on the floor.
continued on page 38
吃蓝莓
L DESIGN IDEAS
is deceptively simple in appearance. In particular, the current step must have an exceptionally fast, high-fidelity transition and faithful turn-on time determination requires substantial measurement bandwidth.
Detailed Measurement Scheme分镜头脚本范例
A more detailed measurement scheme appears in Figure 4. Necessary per-formance parameters for various elements are called out. A subnano-cond ri time pul generator, 1A, 2ns ri time amplifier and a 1GHz oscilloscope are required. The speci-fications reprent realistic operating conditions; other currents and ri times can be lected by altering ap-propriate parameters.
The pul amplifier necessitates careful attention to circuit configura-tion and layout. Figure 5 shows the amplifier includes a paralleled, Dar-lington driven RF transistor output stage. The collector voltage adjustment (“ri time trim”) peaks Q4 to Q6 FT; an input RC network optimizes output pul purity by slightly retarding input pul ri time to within amplifier passband. Paralleling allows Q4 to Q6
to operate at favorable individual cur-rents, maintaining bandwidth. When the (mildly interactive) edge purity and ri time trims are optimized, Figure 6 indicates the amplifier produces a transcendently clean 2ns ri time output pul devoid of ringing, alien components or post-transition excur-sions. Such performance makes diode turn-on time testing practical.1产业园区总体规划
Figure 7 depicts the complete diode forward turn-on time measurement ar-rangement. The pul amplifier, driven by a sub-nanocond pul generator, drives the diode under test. A Z0 probe monitors the measurement point and feeds a 1GHz oscilloscope.2, 3, 4
Diode Testing and Interpreting Results
The measurement test fixture, prop-erly equipped and constructed, permits diode turn-on time testing with excellent time and amplitude resolution.5 Figures 8 through 12 show results for five different diodes from various manufacturers. Figure 8 (Diode Number 1) overshoots steady state forward voltage for 3.6ns, peaking 200mV. This is the best performance of the five. Figures 9 through 12 show increasing turn-on amplitude and time which are detailed in the figure captions. In the worst cas, turn-on amplitudes exceed nominal clamp voltage by more than 1V while turn-on times extend for tens of nanoconds. Figure 12 culminates this unfortunate parade with huge time
and amplitude errors. Such errant excursions can and will cau IC regulator breakdown and failure. The lesson here is clear. Diode turn-on time must be characterized and measured in any given application to insure reliability. L
Notes
1 An alternate pul generation approach appears in Linear Technology Application Note 122, Appendix F, “Another Way to Do It.”
2 Z0 probes are described in Linear Technology Ap-plication Note 122 Appendix C, “About Z0 Probes.” See also References 27 thru 34.3
The subnanocond pul generator requirement is not trivial. See Linear Technology Application Note 122 Appendix B, “Subnanocond Ri Time Pul Generators For The Rich and Poor.”
群雄纷争4 See Linear Linear Technology Application Note 122 Appendix E, “Connections, Cables, Adapters, Attenuators, Probes and Picoconds” for relevant commentary.
5 See Linear Technology Application Note 122 Ap-pendix A, “How Much Bandwidth is Enough?” for discussion on determining necessary measurement bandwidth.
amount of ambient noi in the room. Figure 3 shows the noi spectrum in the chamber without any devices run-ning. This can be ud to determine the actual noi produced by the DUT. Figure 4 shows the worst ca LTM8032 emissions plot, which oc-curs at maximum power out, 10V at
907050E M I S S I O N S L E V E L (d B µV /m )
3010806040200
–10
100200300400500FREQUENCY (MHz)
6007008009001000
EN55022CLASS B
LIMIT研究报告模板
907050E M I S S I O N S L E V E L (d B µV /m )
3010806040200–10
100200300400500FREQUENCY (MHz)
600700800
9001000
EN55022CLASS B LIMIT Figure 3. The baline measurement of ambient noi in the 5-meter chamber (no devices operating)
Figure 4. The LTM8032 emissions for 20W out, 36V IN
2A, from the maximum input voltage, 36V. There are two traces in the plot, one for the vertical and horizontal orientations of the test lab’s receiver antenna. As shown in the figure, the LTM8032 easily meets the CISPR 22 class B limits, with 20db of margin for most of the frequency spectrum, with either antenna orientation.
Conclusion
The LTM8032 switching step-down regulator is both easy to u and quiet, meeting the radiated emissions re-quirements of CISPR22 and EN55022 class B by a wide margin. L
LTM8032, continued from page 33
