LT3684
OPERATION
The LT3684 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 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 cir-
cuitry. The bias regulator normally draws power from the
V
IN
pin, but if the BIAS 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 efficiency. The RUN/SS pin is used to place
the LT3684 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 efficient
operation.
The oscillator reduces the LT3684’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 LT3684 contains a power good comparator which trips
when the FB pin is at 90% 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 LT3684 is
enabled and V
IN
is above 3.6V.
3684f
9
LINER [ LINEAR TECHNOLOGY ]
