LTM8050
12
8050fc
For more information www.linear.com/LTM8050
APPLICATIONS INFORMATION
For most applications, the design process is straight
forward, summarized as follows:
1. Look at Table 1 and find the row that has the desired
input range and output voltage.
2. Apply the recommended C
IN
, C
OUT
, R
FB
and R
T
values.
3. Connect BIAS as indicated.
While these component combinations have been tested
for proper operation, it is incumbent upon the user to
verify proper operation over the intended system’s line,
load and environmental conditions. Bear in mind that the
maximum output current is limited by junction tempera-
ture, the relationship between the input and output voltage
magnitude and polarity and other factors. Please refer to
the graphs in the Typical Performance Characteristics
section for guidance.
The maximum frequency (and attendant R
T
value) at
which the LTM8050 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.
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.
Ceramic capacitors are also piezoelectric. In Burst Mode
operation, the LTM8050’s switching frequency depends
on the load current, and can excite a ceramic capacitor
at audio frequencies, generating audible noise. Since the
LTM8050 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 perfor-
mance electrolytic
capacitor at the output. It may also be
a parallel combination of a ceramic capacitor and a low
cost electrolytic capacitor.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8050. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8050 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.
Frequency Selection
The LTM8050 uses a constant frequency PWM architec-
ture that can be programmed to switch from 100kHz 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 re-
sultant frequencies.
