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LTM8031MPV#PBF

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LTM8031
10
8031fb
applicaTions inForMaTion
may have only a small fraction of their nominal capaci-
tance resulting in much higher output voltage ripple than
expected. Ceramic capacitors are also piezoelectric. In
Burst Mode operation, the LTM8031’s switching frequency
depends on the load current, and can excite a ceramic
capacitor at audio frequencies, generating audible noise.
Since the LTM8031 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet to a
casual ear. If this audible noise is unacceptable, use a high
performance electrolytic capacitor at the output. The input
capacitor can be a parallel combination of a 1µF ceramic
capacitor and a low cost electrolytic capacitor.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8031. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8031 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possi-
bly exceeding the device’s rating. This situation is easily
avoided; see the Hot-Plugging Safely section.
Electromagnetic Compliance
The LTM8031 is compliant with the radiated emissions
requirements of EN55022 class B. Graphs of the LTM8031’s
EMC performance are given in the Typical Performance
Characteristics section. Further data, operating conditions
and test setup are detailed in an EMI Test report available
from Linear Technology.
Frequency Selection
The LTM8031 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.4MHz
by using a resistor tied from the RT pin to ground. Table 2
provides a list of R
T
resistor values and their resultant
frequencies.
Operating Frequency Trade-Offs
It is recommended that the user apply the optimal R
T
value given in Table 1 for the input and output operating
condition. System level or other considerations, however,
may necessitate another operating frequency. While the
LTM8031 is flexible enough to accommodate a wide range
of operating frequencies, a haphazardly chosen one may
result in undesirable operation under certain operating or
fault conditions. A frequency that is too high can reduce
efficiency, generate excessive heat or even damage the
LTM8031 if the output is overloaded or short-circuited. A
frequency that is too low can result in a final design that
has too much output ripple or unnecessarily large output
capacitor. The maximum frequency (and attendant R
T
value)
at which the LTM8031 should be allowed to switch is given
in Table 1 in the f
MAX
column, while the recommended
frequency (and R
T
value) for optimal efficiency over the
given input condition is given in the f
OPTIMAL
column.
There are additional conditions that must be satisfied if
the synchronization function is used. Please refer to the
Synchronization section for details.
Table 2. Switching Frequency vs R
T
Value
SWITCHING FREQUENCY (MHz) R
T
VALUE (kΩ)
0.2 187
0.3 124
0.4 88.7
0.5 69.8
0.6 56.2
0.7 47.5
0.8 39.2
0.9 34
1.0 28.0
1.2 23.7
1.4 19.1
1.5 16.2
1.8 13.3
2 11.5
2.2 9.76
2.4 8.66
LTM8031
11
8031fb
applicaTions inForMaTion
BIAS Pin Considerations
The BIAS pin is used to provide drive power for the internal
power switching stage and operate internal circuitry. For
proper operation, it must be powered by at least 2.8V. If
the output voltage is programmed to be 2.8V or higher,
simply tie BIAS to AUX. If V
OUT
is less than 2.8V, BIAS
can be tied to V
IN
or some other voltage source. In all
cases, ensure that the maximum voltage at the BIAS pin
is both less than 25V and the sum of V
IN
and BIAS is less
than 56V. If BIAS power is applied from a remote or noisy
voltage source, it may be necessary to apply a decoupling
capacitor locally to the LTM8031.
Load Sharing
Two or more LTM8031s may be paralleled to produce higher
currents. This may, however, alter the EMI performance of
the LTM8031s. To do this, tie the V
IN
, ADJ, V
OUT
and SHARE
pins of all the paralleled LTM8031s together. To ensure that
paralleled modules start up together, the RUN/SS pins may
be tied together, as well. Synchronize the LTM8031s to an
external clock to eliminate beat frequencies, if required.
If the RUN/SS pins are not tied together, make sure that
the same valued soft-start capacitors are used for each
module. An example of two LTM8031 modules configured
for load sharing is given in the Typical Applications sec-
tion. For 2A applications also see the LTM8032, 2A EMC
DC/DC µModule regulator
Burst Mode Operation
To enhance efficiency at light loads, the LTM8031 auto-
matically switches to Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LTM8031 delivers single cycle bursts of
current to the output capacitor followed by sleep periods
where the output power is delivered to the load by the output
capacitor. In addition, V
IN
and BIAS quiescent currents are
reduced to typically 25µA and 60µA respectively during
the sleep time. As the load current decreases towards a
no-load condition, the percentage of time that the LTM8031
operates in sleep mode increases and the average input
current is greatly reduced, resulting in higher efficiency.
Burst Mode operation is enabled by tying SYNC to GND. To
disable Burst Mode operation, tie SYNC to a stable voltage
above 0.7V. Do not leave the SYNC pin floating.
Minimum Input Voltage
The LTM8031 is a step-down converter, so a minimum
amount of headroom is required to keep the output in regu-
lation. In addition, the input voltage required to turn on is
higher than that required to run, and depends upon whether
the RUN/SS is used. As shown in the Typical Performance
Characteristics section, it takes only about 3.6V
IN
for the
LTM8031 to run a 3.3V output at light load. If RUN/SS is
pulled up to V
IN
, it takes 5.7V
IN
to start. If the LTM8031
is enabled via the RUN/SS pin, the minimum voltage to
start at light loads is lower, about 4.4V. Similar curves for
2.5V
OUT
, 5V
OUT
and 8V
OUT
operation are also provided in
the Typical Performance Characteristics section.
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8031,
reducing the maximum input current during start-up. The
RUN/SS pin is driven through an external RC network to
create a voltage ramp at this pin. Figure 1 shows the start-
up and shutdown waveforms with the soft-start circuit. By
choosing an appropriate RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply at least 20µA when
the RUN/SS pin reaches 2.5V.
LTM8031
12
8031fb
Synchronization
The internal oscillator of the LTM8031 can be synchro-
nized by applying an external 250kHz to 2MHz clock to
the SYNC pin. Do not leave this pin floating. The resistor
tied from the RT pin to ground should be chosen such
that the LTM8031 oscillates 20% lower than the intended
synchronization frequency (see the Frequency Selection
section). The LTM8031 will not enter Burst Mode operation
while synchronized to an external clock, but will instead
skip pulses to maintain regulation.
Shorted Input Protection
Care needs to be taken in systems where the output will
be held high when the input to the LTM8031 is absent.
This may occur in battery charging applications or in
battery back-up systems where a battery or some other
supply is diode ORed with the LTM8031’s output. If the
V
IN
pin is allowed to float and the RUN/SS pin is held high
(either by a logic signal or because it is tied to V
IN
), then
the LTM8031’s internal circuitry will pull its quiescent
current through its internal power switch. This is fine if
your system can tolerate a few milliamps in this state. If
you ground the RUN/SS pin, the internal switch current
will drop to essentially zero. However, if the V
IN
pin is
grounded while the output is held high, then parasitic
diodes inside the LTM8031 can pull large currents from
the output through the V
IN
pin, potentially damaging the
applicaTions inForMaTion
Figure 2. The Input Diode Prevents a Shorted Input from
Discharging a Back-Up Battery Tied to the Output. It Also
Protects the Circuit from a Reversed Input. The LTM8031
Runs Only When the Input is Present
V
OUT
V
IN
RUN/SS
BIAS
R
T
ADJ
LTM8031
8031 F02
V
OUT
GND
V
IN
AUX
SYNC
device. Figure 2 shows a circuit that will run only when
the input voltage is present and that protects against a
shorted or reversed input.
PCB Layout
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8031. The LTM8031 is neverthe-
less a switching power supply and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, you may fail to achieve specified
operation with a haphazard or poor layout. See Figure 3
for a suggested layout.
Ensure that the grounding and heat sinking are acceptable.
A few rules to keep in mind are:
1. Place the R
ADJ
and R
T
resistors as close as possible to
their respective pins.
2. Place the C
IN
capacitor as close as possible to the V
IN
and GND connection of the LTM8031. If a capacitor
is connected to the FIN terminals, place it as close
as possible to the FIN terminals, such that its ground
connection is as close as possible to that of the C
IN
capacitor.
3. Place the C
OUT
capacitor as close as possible to the
V
OUT
and GND connection of the LTM8031.
8031 F01
I
L
0.5A/DIV
V
RUN/SS
2V/DIV
V
OUT
2V/DIV
RUN/SS
GND
0.22µF
RUN
15k
2ms/DIV
Figure 1. To Soft-Start the LTM8031, Add a Resistor
and Capacitor to the RUN/SS Pin
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LTM8031MPV#PBF

Mfr. #:
Buy LTM8031MPV#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Ultralow EMI, 36V, 1A Step-down Module Regulator
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