LT3695 Series
11
3695fa
OPERATION
The LT3695 series are constant-frequency, current mode
step-down regulators. An oscillator, with frequency set by
R
T
, 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 (LT3695) or through an internal resistor divider con-
nected to the output voltage (LT3695-3.3, LT3695-5), 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 (LT3695) or if the output voltage connected to the
OUT 1 and OUT2 pins exceeds 3V (LT3695-3.3, LT3695-5),
bias power will be drawn from the external source. This
improves effi ciency. The RUN/SS pin is used to place the
LT3695 regulators 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 the internal boost
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 LT3695 regulators au-
tomatically switch 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 LT3695 regulators’ operating
frequency when the voltage at the FB pin (LT3695) or the
OUT1,2 pins (LT3695-3.3, LT3695-5) is low. This frequency
foldback helps to control the output current during start-up
and overload conditions.
Internal circuitry monitors the current fl owing through the
catch diode via the DA pin and delays the generation of
new switch pulses if this current is too high (above 1.6A
nominal). This mechanism also protects the part during
short-circuit and overload conditions by keeping the cur-
rent through the inductor under control.
The LT3695 regulators contain a power good comparator
which trips when the FB pin (LT3695) or the OUT1,2 pins
(LT3695-3.3, LT3695-5) are at 90% of their 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 LT3695 regulators are enabled and V
IN
is above the
minimum input voltage.
The LT3695 regulators have an overvoltage protection fea-
ture which disables switching action when V
IN
goes above
38V (typical) during transients. The LT3695 regulators can
then safely sustain transient input voltages up to 60V.
