LTC3406B-2 Datasheet, PDF(6/16 Page) Linear Technology – 2.25MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

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LTC3406B-2 Datasheet, PDF (6/16 Pages) Linear Technology – 2.25MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

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LTC3406B-2

FU CTIO AL DIAGRA

SLOPE

COMP

OSC

FREQ

SHIFT

VFB

0.6V

â EA

VIN

RUN

0.6V REF

0.65V

O+VDET

SHUTDOWN

ICOMP

RS LATCH

SWITCHING

LOGIC

AND

BLANKING

CIRCUIT

ANTI-

SHOOT-

THRU

IRCMP

4 VIN

5â¦

3 SW

2 GND

3406B BD

OPERATIO (Refer to Functional Diagram)

Main Control Loop

The LTC3406B-2 uses a constant frequency, current mode

step-down architecture. Both the main (P-channel

MOSFET) and synchronous (N-channel MOSFET) switches

are internal. During normal operation, the internal top power

MOSFET is turned on each cycle when the oscillator sets

the RS latch, and turned off when the current comparator,

ICOMP, resets the RS latch. The peak inductor current at

which ICOMP resets the RS latch, is controlled by the output

of error amplifier EA. When the load current increases, it

causes a slight decrease in the feedback voltage, FB, rela-

tive to the 0.6V reference, which in turn, causes the EA

amplifierâs output voltage to increase until the average

inductor current matches the new load current. While the

top MOSFET is off, the bottom MOSFET is turned on until

either the inductor current starts to reverse, as indicated by

the current reversal comparator IRCMP, or the beginning of

the next clock cycle.

The comparator OVDET guards against transient over-

shoots >6.25% by turning the main switch off and keeping

it off until the fault is removed.

Pulse Skipping Mode Operation

At light loads, the inductor current may reach zero or re-

verse on each pulse. The bottom MOSFET is turned off by

the current reversal comparator, IRCMP, and the switch

voltage will ring. This is discontinuous mode operation,

and is normal behavior for the switching regulator. At very

light loads, the LTC3406B-2 will automatically skip pulses

in pulse skipping mode operation to maintain output regu-

lation. Refer to LTC3406 data sheet if Burst Mode opera-

tion is preferred.

Short-Circuit Protection

When the output is shorted to ground, the frequency of the

oscillator is reduced to about 310kHz, 1/7 the nominal

sn3406b2 3406b2fs

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