LTC3406B データシートの表示（PDF） - Linear Technology

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LTC3406B

1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

Linear Technology 

LTC3406B

W

FU CTIO AL DIAGRA

SLOPE

COMP

OSC

OSC

VFB/VOUT

5

LTC3406B-1.5

R1 + R2 = 550k

LTC3406B-1.8

R1 + R2 = 540k

FREQ

SHIFT

+

R1

0.6V

FB

– EA

R2

VIN

–

OV

RUN

OVDET

1

0.6V REF 0.6V + ∆VOVL +

SHUTDOWN

ICOMP

SQ

RQ

RS LATCH

SWITCHING

LOGIC

AND

BLANKING

CIRCUIT

ANTI-

SHOOT-

THRU

IRCMP

4 VIN

5Ω

3 SW

2 GND

3406B BD

U

OPERATIO (Refer to Functional Diagram)

Main Control Loop

The LTC3406B 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 com-

parator, 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, relative to the 0.6V reference, which in turn,

causes the EA amplifier’s output voltage to increase until

the average inductor current matches the new load cur-

rent. 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 overshoots >7.8% by

turning the main switch off and keeping it off until the fault

is removed.

6

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 will automatically skip pulses in

pulse skipping mode operation to maintain output regula-

tion. Refer to LTC3406 data sheet if Burst Mode operation

is preferred.

Short-Circuit Protection

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

oscillator is reduced to about 210kHz, 1/7 the nominal

frequency. This frequency foldback ensures that the in-

ductor current has more time to decay, thereby preventing

runaway. The oscillator’s frequency will progressively

increase to 1.5MHz when VFB or VOUT rises above 0V.

3406bfa

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