LT3973IMSE-3.3 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3973IMSE-3.3

42V, 750mA Step-Down Regulator with 2.5µA Quiescent Current and Integrated Diodes

Linear Technology 

LT3973/LT3973-3.3/LT3973-5

OPERATION

The LT3973 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 VIN and SW pins, turning the switch-off when

this current reaches a level determined by the voltage at

VC (see Block Diagram). An error amplifier measures the

output voltage through an external resistor divider tied to

the FB 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 1.15A 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 VIN pin, but if the 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.

If the EN/UVLO pin is low, the LT3973 is shut down and draws

0.75µA from the input. When the EN/UVLO pin exceeds

1.19V, the switching regulator will become active. Under-

voltage lockout is programmable via this pin.

The switch driver operates from either VIN 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

bipolar NPN power switch for efficient operation.

To further optimize efficiency, the LT3973 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 1.8µA.

If the input voltage decreases towards the programmed

output voltage, the LT3973 will start to skip switch-off times

and decrease the switching frequency to maintain output

regulation up to a maximum duty cycle of approximately

97.5%. When the OUT pin is tied to VOUT, the LT3973

regulates the output such that it stays more than 530mV

below VIN; this sets a minimum dropout voltage. This

enforced minimum dropout voltage limits the duty cycle

and keeps the boost capacitor charged during dropout

conditions. Since sufficient boost voltage is maintained,

the internal switch can fully saturate yielding good dropout

performance.

The LT3973 contains a power good comparator which

trips when the FB pin is at 90% of its regulated value. The

PG output is an open-drain 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 LT3973 is

enabled and VIN is above 4.2V.

For more information www.linear.com/LT3973

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