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LT3684

3684f

The LT3684 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 cir-

cuitry. The bias regulator normally draws power from the

VIN pin, but if the BIAS 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 LT3684 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 LT3684’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 LT3684 contains a power good comparator which trips

when the FB pin is at 90% 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 LT3684 is

enabled and VIN is above 3.6V.

OPERATION

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