Datasheet

LTM8025
12
8025fc
For more information www.linear.com/LTM8025
applicaTions inForMaTion
Table 2. Switching Frequency vs R
T
Value
SWITCHING FREQUENCY R
T
VALUE
0.2MHz 215kΩ
0.3MHz 137kΩ
0.4MHz 100kΩ
0.5MHz 76.8kΩ
0.6MHz 63.4kΩ
0.7MHz 52.3kΩ
0.8MHz 44.2kΩ
0.9MHz 38.3kΩ
1MHz 34.0kΩ
1.2MHz 26.7kΩ
1.4MHz 21.5kΩ
1.6MHz 17.8kΩ
1.8MHz 14.7kΩ
2MHz 12.1kΩ
2.2MHz 9.76kΩ
2.4MHz 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
cir
cuitr
y. 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
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 L
TM8025 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
on is higher than that required to run, and depends upon