Datasheet

LT3680
8
Rev C
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The LT3680 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by RT,
enables an RS flip-flop, turning on the internal power
switch. An amplifier and comparator monitor the current
flowing 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 amplifier measures the output
voltage through an external resistor divider tied to the
FB pin and servos the V
C
pin. If the error amplifier’s out-
put 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 cir-
cuitry. The bias regulator normally draws power from the
V
IN
pin, but if the BD pin is connected to an external volt-
age higher than 3V bias power will be drawn from the
external source (typically the regulated output voltage).
This improves efficiency. The RUN/SS pin is used to place
the LT3680 in shutdown, disconnecting the output and
reducing the input current to less than 0.5µA.
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 efficient operation.
To further optimize efficiency, the LT3680 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 75µA in a typical application.
The oscillator reduces the LT3680’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 LT3680 contains a power good comparator which
trips when the FB pin is at 91% 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 LT3680
is enabled and V
IN
is above 3.6V.
OPERATION
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