7

LTC3404

3404fb

+

–

+

–

+

–

+

–

OVDET

EA

+

–

8

7

3

1

RUN

PLL LPF

VCO

X

Y

Y = “0” ONLY WHEN X IS A CONSTANT “1”

BURST

DEFEAT

SLOPE

COMP

OSC

SYNC/MODE

0.6V

FREQ

SHIFT

0.8V

0.85V

0.8V REF

SHUTDOWN

0.55V

0.8V

SLEEP

SLEEP

EN

BURST

2

S

R

RS LATCH

SWITCHING

LOGIC

AND

BLANKING

CIRCUIT

ANTI-

SHOOT-

THRU

Q

Q

6

Ω

6

SW

5

GND

3404 BD

4

P-CH

P-CH

N-CH

FU CTIO AL DIAGRA

OPERATIO

Main Control Loop

The LTC3404 uses a constant frequency, current mode

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

FET) and synchronous (N-channel MOSFET) switches are

internal. During normal operation, the internal top power

MOSFET is turned on each clock cycle when the oscillator

sets the RS latch, and turned off when the current com-

section, allows EA to receive an output feedback voltage

from an external resistive divider. When the load current

increases, it causes a slight decrease in the feedback

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 next clock cycle.

Comparator OVDET guards against transient overshoots

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

until the fault is removed.

Burst Mode Operation

The LTC3404 is capable of Burst Mode operation in which

the internal power MOSFETs operate intermittently based

on load demand. To enable Burst Mode operation, simply

enable PWM pulse skipping mode, connect the SYNC/

MODE pin to GND. In this mode, the efficiency is lower at

light loads, but becomes comparable to Burst Mode

operation when the output load exceeds 50mA. The ad-

vantage of pulse skipping mode is lower output ripple and

less interference to audio circuitry.