LTM8029 LINER [Linear Technology], LTM8029 Datasheet - Page 10

• Manufacturer Part Number: LTM8029
• Description: 36VIN, 600mA Step-Down ?Module Converter
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
LTM8029
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
2. Apply the recommended C
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
which the LTM8029 should be allowed to switch is given in
Table 1 in the f
quency (and R
input condition is given in the f
The LTM8029 is capable of operating at low input voltages
by skipping off-times to maintain regulation. This results
in a lower operating frequency than that programmed by
the RT pin, so it may be necessary to use larger input and
output capacitors, depending upon the system require-
ments. The recommended components and V
listed in Table 1 reflect an operation where off-times are
not skipped.
10
input range and output voltage.
T
MAX
value) for optimal efficiency over the given
column, while the recommended fre-
IN
, C
OPTIMAL
OUT
, R
column.
FB
and R
T
value) at
T
IN
values.
range
Capacitor Selection Considerations
The C
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. 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 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.
Ceramic capacitors are also piezoelectric. In Burst Mode
operation, the LTM8029’s switching frequency depends
on the load current, and can excite a ceramic capacitor
at audio frequencies, generating audible noise. Since the
LTM8029 operates at a lower current limit during Burst
Mode operation, the noise is typically very quiet to a ca-
sual ear. If this audible noise is unacceptable, use a high
performance 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 LTM8029. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8029 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 Safety section.
IN
and C
OUT
capacitor values in Table 1 are the
8029f