Datasheet

LTM8055/LTM8055-1
11
Rev C
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APPLICATIONS INFORMATION
gives the description and part numbers of electrolytic
capacitors used in the LTM8055/LTM8055-1 development
testing and design validation.
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
it is 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 and ap
-
plied voltage
and give dependable service. Other types,
including Y5V and Z5U have very large temperature and
voltage coefficients of capacitance. In an application cir
-
cuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
than expected.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8055/
LTM8055-1. A ceramic input capacitor combined with
trace
or cable inductance forms a high Q (underdamped)
tank circuit. If the LTM8055/LTM8055-1 circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the device’s rating.
This situation is easily avoided; see the Hot-Plugging
Safely section.
Frequency Selection
The LTM8055/LTM8055-1 uses a constant frequency
PWM architecture that can be programmed to switch from
100kHz to 800kHz by tying a resistor from the RT pin to
ground. Table 3 provides a list of R
T
resistor values and
their re-sultant frequencies.
Table 3. Switching Frequency vs R
T
Value
FREQUENCY R
T
VALUE (kΩ)
100 453
200 147
300 84.5
400 59
500 45.3
600 36.5
700 29.4
800 24.9
An external resistor from RT to GND is required. Do not
leave this pin open, even when synchronizing to an ex-
ternal clock. When synchronizing the switching of the
LTM8055/LTM8055-1 to an external signal source, the
frequency range is 200kHz to 700kHz.
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
LTM8055/LTM8055-1 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 LTM8055/LTM8055-1 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, too large of an output capacitor or is unstable.
Parallel Operation
Tw o or more LTM8055/LTM8055-1s may be combined to
provide increased output current
by configuring them as a
master and a slave, as shown in Figure 1. Each LTM8055/
LTM8055-1 is equipped with an I
OUTMON
and a CTL pin.
The I
OUTMON
pin’s 0 to 1.2V signal reflects the current
passing through the output sense resistor, while a voltage
less than 1.2V applied to the CTL pin will limit the current
passing through the output sense resistor. By applying
the voltage of the master’s I
OUTMON
pin to the slave’s CTL
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