LT3685E Ver la hoja de datos (PDF) - Linear Technology

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LT3685E

36V, 2A, 2.4MHz Step-Down Switching Regulator

Linear Technology 

LT3685

OPERATION

The LT3685 is a constant frequency, current mode step-

down regulator. An oscillator, with frequency set by RT,

enables 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. An error amplifier measures the output

voltage through an external resistor divider tied to the FB

pin and servos the VC pin. If the error amplifier’s output

increases, more current is delivered to the output; if it

decreases, less current is delivered. An active clamp on the

VC pin provides current limit. The VC pin is also clamped to

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 VIN pin,

but if the BD pin is connected to an external voltage higher

than 3V bias power will be drawn from the external source

(typically the regulated output voltage). This improves

efficiency. The RUN/SS pin is used to place the LT3685

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 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 efficient operation.

The oscillator reduces the LT3685’s operating frequency

when the voltage at the FB pin is low. This frequency

foldback helps to control the output current during startup

and overload.

The LT3685 contains a power good comparator which trips

when the FB pin is at 89% 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 LT3685 is

enabled and VIN is above 3.6V.

The LT3685 has an overvoltage protection feature which

disables switching action when the VIN goes above 38V

typical (36V minimum). When switching is disabled, the

LT3685 can safely sustain input voltages up to 60V.

3685fb

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