LT3668 datasheet(12/28 Pages) LINER

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LT3668

3668fa

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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 RT, 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-

cuitry. 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

theexternalsource(typicallytheregulatedoutputvoltage).

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

pullingtheEN2/ILIM2orEN3/ILIM3pinhighshutsdownthe

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.

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