lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 11

• Manufacturer Part Number: lt3695
• Description: 1a Fault Tolerant Micropower Step-down Regulator
• Manufacturer: Linear Technology Corporation

OPERATION
The LT3695 is a constant frequency, current mode step-
down regulator. An oscillator, with frequency set by R
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
off when this current reaches a level determined by the
voltage at V
voltage through an external resistor divider tied to the FB
pin and servos the V
increases, more current is delivered to the output; if it
decreases, less current is delivered. An active clamp on the
V
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
pin, but if the BD pin is connected to an external voltage
higher than 3V, bias power will be drawn from the external
source. This improves effi ciency. The RUN/SS pin is used
to place the LT3695 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.
C
pin provides current limit. The V
C
. An error amplifi er measures the output
C
IN
pin. If the error amplifi er’s output
and SW pins, turning the switch
C
pin is also clamped to
T
IN
,
To further optimize effi ciency, the LT3695 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 LT3695’s operating frequency
when the voltage at the FB pin is low. This frequency fold-
back 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 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 LT3695 is
enabled and V
The LT3695 has an overvoltage protection feature which
disables switching action when V
cal) during transients. The LT3695 can then safely sustain
transient input voltages up to 60V.
IN
is above the minimum input voltage.
IN
goes above 38V (typi-
LT3695
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