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LTM8026
APPLICATIONS INFORMATION
graphs in the Typical Performance Characteristics section
for guidance.
The maximum frequency (and attendant R T value) at
which the LTM8026 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
Switching Frequency Synchronization section for details.
Capacitor Selection Considerations
The C IN and C OUT capacitor values in Table 1 are the
minimum recommended values for the associated oper-
ating conditions. Applying capacitor values below those
indicated in Table 1 is not recommended, and may result
in undesirable operation. Using larger values is generally
acceptable, and can yield improved dynamic response, if
necessary. Again, it is incumbent upon the user to verify
proper operation over the intended system’s line, load and
environmental conditions.
Ceramic capacitors are small, robust and have very low ESR.
However, not all ceramic capacitors are suitable. X5R and
X7R types are stable over temperature, applied voltage and
give dependable service. Other types, including Y5V and
Z5U have very large temperature and voltage coefficients
of capacitance. In an application circuit they may have only
a small fraction of their nominal capacitance resulting in
much higher output voltage ripple than expected.
Many of the output capacitances given in Table 1 specify
an electrolytic capacitor. Ceramic capacitors may also be
used in the application, but it may be necessary to use
more of them. Many high value ceramic capacitors have a
large voltage coefficient, so the actual capacitance of the
component at the desired operating voltage may be only
a fraction of the specified value. Also, the very low ESR of
ceramic capacitors may necessitate additional capacitors
for acceptable stability margin.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8026. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8026 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.
Programming Switching Frequency
The LTM8026 has an operational switching frequency
range between 100kHz and 1MHz. This frequency is
programmed with an external resistor from the RT pin to
ground. Do not leave this pin open under any circumstance.
See Table? 2 for resistor values and the corresponding
switching frequencies.
Table 2. R T Resistor Values and Their Resultant Switching
Frequencies
SWITCHING FREQUENCY (MHz) R T (kΩ)
1 39.2
0.750 53.6
0.5 82.5
0.3 140
0.2 210
0.1 453
In addition, the Typical Performance Characteristics sec-
tion contains a graph that shows the switching frequency
versus R T value.
To improve efficiency at light load, the part will enter
discontinuous mode.
Switching 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
LTM8026 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
LTM8026 in some fault conditions. A frequency that is too
low can result in a final design that has too much output
ripple or too large of an output capacitor.
Switching Frequency Synchronization
The nominal switching frequency of the LTM8026 is
determined by the resistor from the RT pin to GND and
8026fb
14
For more information www.linear.com/LTM8026
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