LT3480
8
3480fb
OPERATION
The LT3480 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
enables an RS fl ip-fl op, turning on the internal power
switch. An amplifi er and comparator monitor the current
fl owing between the V
IN
and SW pins, turning the switch
off when this current reaches a level determined by the
voltage at V
C
. An error amplifi er measures the output
voltage through an external resistor divider tied to the FB
pin and servos the V
C
pin. If the error amplifi er’s output
increases, more current is delivered to the output; if it
decreases, less current is delivered. An active clamp on the
V
C
pin provides current limit. The V
C
pin is also clamped to
the voltage on the RUN/SS pin; soft-start is implemented
by generating a voltage ramp at the RUN/SS pin using an
external resistor and capacitor.
An internal regulator provides power to the control circuitry.
The bias regulator normally draws power from the V
IN
pin,
but if the BD pin is connected to an external voltage higher
than 3V bias power will be drawn from the external source
(typically the regulated output voltage). This improves
effi ciency. The RUN/SS pin is used to place the LT3480
in shutdown, disconnecting the output and reducing the
input current to less than 1μA.
The switch driver operates from either the input or from
the BOOST pin. An external capacitor and diode are used
to generate a voltage at the BOOST pin that is higher than
the input supply. This allows the driver to fully saturate the
internal bipolar NPN power switch for effi cient operation.
To further optimize effi ciency, the LT3480 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling
the output switch is shut down, reducing the input supply
current to 70μA in a typical application.
The oscillator reduces the LT3480’s operating frequency when
the voltage at the FB pin is low. This frequency foldback helps
to control the output current during startup and overload.
The LT3480 contains a power good comparator which trips
when the FB pin is at 86% of its regulated value. The PG
output is an open-collector transistor that is off when the
output is in regulation, allowing an external resistor to pull
the PG pin high. Power good is valid when the LT3480 is
enabled and V
IN
is above 3.6V.
The LT3480 has an overvoltage protection feature which
disables switching action when the V
IN
goes above 38V
typical (36V minimum). When switching is disabled, the
LT3480 can safely sustain input voltages up to 60V.
