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LTM8025 Datasheet(PDF) 13 Page - Linear Technology

Part No. LTM8025
Description  36V, 3A Step-Down Module Regulator
Download  22 Pages
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LTM8025 Datasheet(HTML) 13 Page - Linear Technology

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LTM8025
13
8025fc
For more information www.linear.com/LTM8025
applicaTions inForMaTion
Figure 1. To Soft-Start the LTM8025, Add a Resistor and
Capacitor to the RUN/SS Pin
whether the RUN/SS is used. As shown in the Typical
Performance Characteristics section, the minimum input
voltage to run a 3.3V output at light load is only about 3.6V,
but, if the RUN/SS is pulled up to VIN, it takes 5.5VIN to
start. If the LTM8025 is enabled with the RUN/SS pin, the
minimum voltage to start at light loads is lower, about 4.3V.
Similar curves detailing this behavior of the LTM8025 for
other outputs are also included in the Typical Performance
Characteristics section.
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8025,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC filter to
create a voltage ramp at this pin, as shown in Figure 1.
By choosing an appropriate RC time constant, the peak
start-up current can be reduced to the current that is re-
quired to regulate the output, with no overshoot. Choose
the value of the resistor so that it can supply at least 20μA
when the RUN/SS pin reaches 2.5V.
Frequency Foldback
8025 F01
RUN/SS
RUN
GND
15k
0.22µF
The LTM8025 is equipped with frequency foldback which
actstoreducethethermalandenergystressontheinternal
power elements during a short circuit or output overload
condition.IftheLTM8025detectsthattheoutputhasfallen
out of regulation, the switching frequency is reduced as a
function of how far the output is below the target voltage.
Thisinturnlimitstheamountofenergythatcanbedelivered
to the load under fault. During the start-up time, frequency
foldback is also active to limit the energy delivered to the
potentially large output capacitance of the load.
Synchronization
TheinternaloscillatoroftheLTM8025canbesynchronized
by applying an external 250kHz to 2MHz clock to the SYNC
pin. Do not leave this pin floating. When synchronizing
the LTM8025, select an RT resistor value that corresponds
to an operating frequency 20% lower than the intended
synchronization frequency (see the Frequency Selection
section).
Inadditiontosynchronization,theSYNCpincontrolsBurst
Mode behavior. If the SYNC pin is driven by an external
clock, or pulled up above 0.7V, the LTM8025 will not en-
ter Burst Mode operation, but will instead skip pulses to
maintain regulation instead.
Shorted Input Protection
Care needs to be taken in systems where the output will be
held high when the input to the LTM8025 is absent. This
may occur in battery charging applications or in battery
backup systems where a battery or some other supply is
diode ORed with the LTM8025’s output. If the VIN pin is
allowed to float and the SHDN pin is held high (either by a
logicsignalorbecauseitistiedtoVIN),thentheLTM8025’s
internal circuitry will pull its quiescent current through
its internal power switch. This is fine if your system can
tolerate a few milliamps in this state. If you ground the
RUN/SS pin, the input current will drop to essentially zero.
However, if the VIN pin is grounded while the output is
held high, then parasitic diodes inside the LTM8025 can
pull large currents from the output through the VIN pin.
Figure 2 shows a circuit that will run only when the input
voltage is present and that protects against a shorted or
reversed input.
PCB Layout
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8025. The LTM8025 is neverthe-
less a switching power supply, and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, you may fail to achieve specified
operation with a haphazard or poor layout. See Figure 3
for a suggested layout. Ensure that the grounding and
heatsinking are acceptable.


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