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LTM8031
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.
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
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.
8031fb
10
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