LTM8025 Datasheet, PDF (12/20 Pages) Linear Technology – 36V, 3A Step-Down μModule Converter

LTM8025
APPLICATIONS INFORMATION
Table 2. Switching Frequency vs R_T Value
SWITCHING FREQUENCY
0.2MHz
0.3MHz
0.4MHz
0.5MHz
0.6MHz
0.7MHz
0.8MHz
0.9MHz
1MHz
1.2MHz
1.4MHz
1.6MHz
1.8MHz
2MHz
2.2MHz
2.4MHz
R_T VALUE
215kΩ
137kΩ
100kΩ
76.8kΩ
63.4kΩ
52.3kΩ
44.2kΩ
38.3kΩ
34.0kΩ
26.7kΩ
21.5kΩ
17.8kΩ
14.7kΩ
12.1kΩ
9.76kΩ
8.25kΩ
Operating Frequency Tradeoffs
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
LTM8025 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
LTM8025 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 or too large of an output
capacitor.
BIAS Pin Considerations
The BIAS pin is used to provide drive power for the in-
ternal power switching stage and operate other internal
circuitry. For proper operation, it must be powered by at
least 2.8V. If the output voltage is programmed to 2.8V
or higher, BIAS may be simply tied to AUX. If V_OUT is less
than 2.8V, BIAS can be tied to V_IN or some other voltage
source. If the BIAS pin voltage is too high, the efficiency
of the LTM8025 may suffer. The optimum BIAS voltage is
12
dependent upon many factors, such as load current, input
voltage, output voltage and switching frequency, but 4V to
5V works well in many applications. In all cases, ensure
that the maximum voltage at the BIAS pin is less than 25V
and that the sum of V_IN and BIAS is less than 56V. If BIAS
power is applied from a remote or noisy voltage source, it
may be necessary to apply a decoupling capacitor locally
to the pin.
Load Sharing
Two or more LTM8025’s may be paralleled to produce higher
currents. To do this, tie the V_IN, ADJ, V_OUT and SHARE
pins of all the paralleled LTM8025’s together. To ensure
that paralleled modules start up together, the RUN/SS pins
may be tied together, as well. If the RUN/SS pins are not
tied together, make sure that the same valued soft-start
capacitors are used for each module. Current sharing can
be improved by synchronizing the LTM8025s. An example
of two LTM8025s configured for load sharing is given in
the Typical Applications section.
Burst Mode Operation
To enhance efficiency at light loads, the LTM8025 auto-
matically switches to Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LTM8025 delivers single cycle bursts of
current to the output capacitor followed by sleep periods
where the output power is delivered to the load by the output
capacitor. In addition, V_IN and BIAS quiescent currents are
each reduced to microamps during the sleep time. As the
load current decreases towards a no load condition, the
percentage of time that the LTM8025 operates in sleep
mode increases and the average input current is greatly
reduced, resulting in higher efficiency.
Burst Mode operation is enabled by tying SYNC to GND. To
disable Burst Mode operation, tie SYNC to a stable voltage
above 0.7V. Do not leave the SYNC pin floating.
Minimum Input Voltage
The LTM8025 is a step-down converter, so a minimum
amount of headroom is required to keep the output in
regulation. In addition, the input voltage required to turn
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