December 2009 - EMI Certified Step-Down Converter in 15mm × 9mm uModule Package Produces 1A, 0.8VOUT –10VOUT from 3.6VIN – 36VIN

DESIGN IDEAS L
EMI Certified Step-Down Converter
in 15mm × 9mm µModule Package
Produces 1A, 0.8VOUT –10VOUT from
3.6VIN – 36VIN
by David Ng
Introduction
VIN
4.75V TO 36V*
The LTM8031 is a low noise DC/DC
step-down µModule regulator that
operates from input voltages from 3.6V
to 36V and is independently certified
to be compliant with the stringent
requirements of EN55022 class B.
Its output can be programmed to
provide voltages from 0.8V to 10V at
up to 1A. A nearly complete converter
solution is contained in the 15mm ×
9mm × 2.82mm package. As seen in
Figure 1, the LTM8031 just requires
one resistor to set the output voltage,
another resistor to set the operating
frequency, and some input and output
capacitance.
FIN
22µF
LTM8031
VOUT
3.3V
1A
AUX
RUN/SS
1µF
BIAS
SHARE
PGOOD
RT SYNC GND ADJ
48.7k
78.7k
*RUNNING VOLTAGE RANGE
Figure 1. The LTM8031 needs just two resistors and some capacitance to get it running.
Table 1. Low noise µModule DC/DC step down converters
LTM8031 Features
The LTM8031 comes with many of
the features often required in modern
power supplies. It can be sequenced
via its RUN/SS pin, which also serves
to allow the user to control the output
behavior at turn on. A PGOOD pin
indicates when the output is within
10% of the target regulation voltage.
The SYNC pin is used to synchronize
the LTM8031 to a desired system
frequency.
OUT
VIN
Product
VIN Range
VOUT Range
ILOAD(MAX)
LTM8020
4V to 36V
1.25V to 5V
200mA
LTM8021
3.6V to 36V
0.8V to 5V
500mA
LTM8031
3.6V to 36V
0.8V to 10V
1A
LTM8032
3.6V to 36V
0.8V to 10V
2A
Through the incorporation of a high
Q pi filter and electromagnetic interference reduction design techniques,
the LTM8031 is purpose built with low
radiated emissions in mind. It has been
independently verified to be compliant with EN55022:2006 (Information
Technology Equipment – Radio Disturbance Characteristics – Limits and
Methods of Measurement). EN55022
has two classes, A and B. The LTM8031
is certified compliant with class B,
the more stringent of the two. More
than that, the LTM8031 beats the
continued on page 38
EMISSIONS LEVEL (dBµV/m)
70
60
50
CISPR22 CLASS B STANDARD
40
30
MEASURED EMISSIONS
20
10
VIN = 36V
VOUT = 10V
ILOAD = 1A
0
–10
0
200
400
600
FREQUENCY (MHz)
TWO EMISSIONS TRACES:
HORIZONTAL AND VERTICAL
ANTENNA ORIENTATIONS
80
800
1000
Figure 2. The LTM8031 meets EN55022 Class
B while producing 10VOUT at 1A from 36VIN.
Linear Technology Magazine • December 2009
70
60
50
CISPR22 CLASS B STANDARD
40
30
MEASURED EMISSIONS
20
10
VIN = 36V
VOUT = 5V
ILOAD = 1A
0
–10
0
200
400
600
FREQUENCY (MHz)
TWO EMISSIONS TRACES:
HORIZONTAL AND VERTICAL
ANTENNA ORIENTATIONS
80
800
1000
Figure 3. The LTM8031 is EN55022 Class B
compliant with 5VOUT at 1A from 36VIN.
EMISSIONS LEVEL (dBµV/m)
TWO EMISSIONS TRACES:
HORIZONTAL AND VERTICAL
ANTENNA ORIENTATIONS
80
90
90
EMISSIONS LEVEL (dBµV/m)
90
70
60
50
CISPR22 CLASS B STANDARD
40
30
MEASURED EMISSIONS
20
10
VIN = 36V
VOUT = 2.5V
ILOAD = 1A
0
–10
0
200
400
600
FREQUENCY (MHz)
800
1000
Figure 4. The LTM8031 beats the EN55022
Class B limits by a wide margin with 2.5VOUT
at 1A from 36VIN.
31
L NEW DEVICE CAMEOS
New Device Cameos
The LTC4215-2 and LTC4215-3 are
low voltage Hot Swap™ controllers with
onboard ADCs and I2C compatible interfaces. Functionally, the LTC4215‑2
and LTC4215-3 are similar to the
LTC4215 and LTC4215-1, respectively, with the overcurrent fault timers
extended from 20µs to 420µs.
Some Hot Swap applications require
more than the 20µs overcurrent fault
filter time provided by the LTC4215
and LTC4215-1. For example, some
loads draw brief surge currents and
fast input voltage steps cause high
transient charging currents into bulk
output capacitance. Increasing the
overcurrent fault filter time from 20µs
to 420µs allows applications to ride out
such current surges, while still turning
off before MOSFET safe operating area
limits have been reached.
LTM8031, continued from page 31
EN55022B limits by a wide margin,
better than 20dB in some bands. This
is shown in Figures 2, 3 and 4, where
the LTM8031 is operated from 36VIN to
produce 10VOUT, 5VOUT and 2.5VOUT
at 1A, respectively. The 36VIN, 10VOUT
at 1A configuration is a worst case
radiated emissions scenario, because
the LTM8031 is operating with both
maximum rated switching voltage and
output power.
EMISSIONS LEVEL (dBµV/m)
50
CISPR22 CLASS B STANDARD
30
MEASURED EMISSIONS
20
10
VIN = 36V
VOUT = 5V
ILOAD = 200mA
0
–10
0
200
400
600
FREQUENCY (MHz)
800
1000
Figure 5. The LTM8020 is our quietest EMC
µModule converter, providing 5V at 200mA
from 36VIN.
38
The LTM8031 is rated to provide
1A, but what about other load currents? Linear Technology offers three
other EN55022 class B compliant
µModule DC/DC converters, as
well: the LTM8020 for 200mA loads,
the LTM8021 for 500mA, and the
LTM8032 for 2A loads. These are
summarized in Table 1, and the worst
case emissions for each are given in
Figures 5, 6, and 7, respectively
70
50
CISPR22 CLASS B STANDARD
30
MEASURED EMISSIONS
20
10
VIN = 36V
VOUT = 5V
ILOAD = 500mA
0
–10
0
200
400
600
FREQUENCY (MHz)
Conclusion
It is not uncommon to have to design a
power system that requires numerous
power rails at different load currents,
low radiated emissions, and small
size. For ease of design, take a look
at the LTM8031 and its family of low
noise µModule DC/DC step-down
converters. L
TWO EMISSIONS TRACES:
HORIZONTAL AND VERTICAL
ANTENNA ORIENTATIONS
80
60
40
of a transceiver. The LTM2881 breaks
ground loops by isolating the logic level
interface and line transceiver using
internal inductive signal isolation that
allows for a much larger common mode
voltage range plus superior common
mode rejection of >30kV/µs. A low
EMI DC/DC converter powers the
transceiver and provides an isolated
5V supply output for powering any
supporting external components.
With 2,500VRMS of galvanic isolation, onboard secondary power and
a fully compliant RS485 transmitter
and receiver, the LTM2881 requires
no external components, ensuring a
complete, compact µModule solution
for isolated serial data communications.
Two versions of the LTM2881 are
available for 3.3V and 5V input supplies, in low profile 11.25 × 15mm ×
2.8mm surface mount LGA and BGA
packages. L
90
TWO EMISSIONS TRACES:
HORIZONTAL AND VERTICAL
ANTENNA ORIENTATIONS
80
60
40
The LTM2881 is an isolated RS485
µModule transceiver that guards
against large ground-to-ground differentials and common mode transients.
In practical RS485 systems, ground
potentials vary widely from node to
node, often exceeding the tolerable
range, which can result in an interruption of communications or destruction
90
TWO EMISSIONS TRACES:
HORIZONTAL AND VERTICAL
ANTENNA ORIENTATIONS
70
Isolated RS485 µModule
Transceiver Integrates
Isolated Power
EMISSIONS LEVEL (dBµV/m)
90
80
The LTC4215-2 also defaults
to auto-retry after experiencing
an overcurrent fault, making the
LTC4215-2 the only part in the
LTC4215 family that will auto-retry by
default after an overcurrent fault.
The LTC4215-2 and LTC4215-3
work in applications from 12V (with
transients to 24V) down to 3.3V and
are available in 4mm × 5mm QFN
packages.
800
1000
Figure 6. The LTM8021 is another EN55022B
compliant device, capable of outputting 5V at
500mA from 36VIN.
EMISSIONS LEVEL (dBµV/m)
Low Voltage Hot Swap
Controllers Provide
Extended Fault Timers
70
60
50
CISPR22 CLASS B STANDARD
40
30
MEASURED EMISSIONS
20
10
VIN = 36V
VOUT = 10V
ILOAD = 2A
0
–10
0
200
400
600
FREQUENCY (MHz)
800
1000
Figure 7. The LTM8032, weighing in at 10VOUT
at 2A from 36VIN, meets EN55022 class B.
Linear Technology Magazine • December 2009
Similar pages