LTM8045
13
8045fc
For more information www.linear.com/LTM8045
applicaTions inForMaTion
Setting Output Voltage
The output voltage is set by connecting a resistor (R
FB
)
from V
OUT
+
to the FB pin for a SEPIC and from V
OUT
–
to
the FB pin for an inverting converter. R
FB
is determined
from the equation R
FB
= [(V
OUT
– 1.215)/0.0833]kΩ for
a SEPIC and from R
FB
= [(|V
OUT
| + 0.005)/0.0833]kΩ for
an inverting converter.
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 LTM8045. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8045 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 LTM8045 has an operational switching frequency range
between 200kHz and 2MHz. The free running frequency is
programmed with an external resistor from the RT pin to
ground. Do not leave this pin open under any circumstance.
When the SYNC pin is driven low (< 0.4V), the frequency
of operation is set by the resistor from RT to ground. The
R
T
value is calculated by the following equation:
R
T
=
f
OSC
− 1
where f
OSC
is the typical switching frequency in MHz and
R
T
is in kΩ.
Switching Frequency Trade-Offs
It is recommended that the user apply the optimal R
T
value
given in Table 1 for the corresponding input and output
operating condition. System level or other considerations,
however, may necessitate another operating frequency.
While the LTM8045 is flexible enough to accommodate a
wide range of operating frequencies, a haphazardly chosen
one may result in undesirable operation under certain op
-
erating or fault conditions. A frequency that is too high can
reduce
efficiency, generate excessive heat or even damage
the LTM8045 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 switching frequency can be synchronized to an external
clock source. To synchronize to the external source, simply
provide a digital clock signal at the SYNC pin. Switching
will occur at the SYNC clock frequency. Drive SYNC low
and the switching frequency will revert to the internal
free-running oscillator after a few clock periods.
Switching will stop if SYNC is driven high.
The duty cycle of
SYNC must be between 35% and 65%
for
proper operation. Also, the frequency of the SYNC
signal must meet the following two criteria:
1. SYNC may not toggle outside the frequency range of
200kHz to 2MHz unless it is stopped low to enable the
free-running oscillator.
2. The SYNC frequency can always be higher than the
free-running oscillator frequency, f
OSC
, but should not
be less than 25% below f
OSC
(f
OSC
is set by R
T
).
