LT3667
15
3667fb
For more information www.linear.com/LT3667
OPERATION
The LT3667 combines a 400mA buck switching regulator
and two 200mA low dropout linear regulators. Operation
is best understood by referring to the Block Diagrams.
The buck regulator part is a constant frequency, current
mode step-down regulator. An oscillator, with frequency
set by R
T
, sets an RS flip-flop, turning on the internal
power switch. An amplifier and comparator monitor the
current flowing between the IN1 and SW pins, turning the
switch off when this current reaches a level determined by
the voltage at VC. An error amplifier measures the output
voltage through an external resistor divider tied to the
FB1 pin and servos the VC node. If the error amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered.
Another comparator monitors the current flowing through
the catch diode and reduces the operating frequency when
the current exceeds the 500mA bottom current limit. This
foldback in frequency helps to control the output current
in fault conditions such as a shorted output with high
input voltage. Maximum deliverable current to the output
is therefore limited by both switch current limit and catch
diode current limit.
An internal regulator provides
power to
the control circuitry.
The bias regulator normally draws power from the IN1 pin,
but if the IN3/BD (MSOP) or BD (QFN) pin is connected
to an external voltage higher than 3.2V, bias power will
be drawn from the external source (typically the regulated
output voltage). This improves efficiency.
The switch driver operates from either IN1 or from the
BOOST pin. An external capacitor is 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
NPN power switch for efficient operation.
To further optimize efficiency, the LT3667 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 50μA (including the current drawn by the LDOs).
The switching regulator has an overvoltage protection
feature which disables switching action when IN1 goes
above 42V (typical) during transients. It can then safely
sustain transient input voltages up to 60V.
The switching regulator can also be separately shut down
via the UVLO1 pin, which can be used to implement a
programmable undervoltage lockout with an external
resistive
divider.
The LDO blocks are micropower, low noise 200mA linear
regulators with low dropout voltage and current limit,
which provide fast transient response with minimum low
ESR 2.2µF ceramic output capacitors. Each output cur
-
rent limit
can be programmed individually with a single
resistor,
and pulling the EN2/ILIM2 or EN3/ILIM3 pin high
shuts down the corresponding LDO. Internal protection
circuitry includes reverse-battery protection, reverse-
output protection, reverse-current protection, and current
limit with foldback.
The internal reference voltage circuitry is supplied by
the IN1 and IN2 pins. This allows the LDO at IN2 to run
independently and supply the switching regulator with its
output OUT2.
The EN pin is used to place the LT3667 in shutdown,
thereby reducing the input current to less than 1μA.
The LT3667 contains three power good window com
-
parators that
indicate whether the output voltages are
within ±10% of their nominal value. The outputs of these
comparators are open-drain transistors which are off
when their corresponding output is in regulation, allowing
external resistors to pull the power good pins high. The
PG pin provides a combined power good signal, while the
QFN package additionally allows access to the individual
power good
signals through
pins PG1, PG2 and PG3.
Power good is valid if the LT3667 is enabled and IN1 or
IN2 are above their minimum input voltages.
Internal thermal limiting protects the LT3667 during
overload conditions.
