LT3668
12
3668fa
For more information www.linear.com/LT3668
OPERATION
The LT3668 combines a 400mA buck switching regulator
and two 200mA low dropout linear tracking regulators.
Operation is best understood by referring to the Block
Diagram.
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 cir-
cuitr
y. The bias regulator normally draws power from the
IN1
pin, but if the IN3/BD 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 LT3668 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 sup
-
ply 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 LDO blocks are micropower, low noise 200mA linear
tracking regulators with low dropout voltage and current
limit, which provide fast transient response with minimum
low ESR 10μF
ceramic output capacitors. The output volt-
age
of each LDO follows a reference voltage applied to its
adjust
input with high accuracy. Each output current 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 LT3668 in shutdown,
thereby reducing the input current to less than 1μA.
The LT3668 contains a power good window comparator
that indicates whether the output voltage of the switching
regulator is within ±10% of its nominal value. The output
PG of this comparator is an open-drain transistor which
is off when the output is in regulation, allowing external
resistors to pull the PG pin high. Power good is valid if
the LT3668 is enabled and IN1 or IN2 are above their
minimum input voltages.
Internal thermal limiting protects the LT3668 during
overload
conditions.
