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LTC3448

3448f

FU CTIO AL DIAGRA

OPERATIO (Refer to Functional Diagram)

–

IRCMP

–

ICOMP

RUN

OSC

SLOPE

COMP

LDO CONTROL

LOGIC

LDO

DRIVE

0.6V

0.6V REF

VIN

FREQ

SYNC

MODE

VOUT

VFB

SHUTDOWN

VIN

OSC

RS LATCH

SWITCHING

LOGIC

AND

BLANKING

CIRCUIT

ANTI-

SHOOT-

THRU

Ω

GND

3448 F01

–

0.6V +

∆OVL

OVDET

Figure 1

Main Control Loop

The LTC3448 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 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 FBINT 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 re-

verse, as indicated by the current reversal comparator

IRCMP, or the beginning of the next clock cycle. The

comparator OVDET guards against transient overshoots

5.8% by turning off the main switch and keeping it off until

the fault is removed.

Pulse Skipping Mode Operation

At light loads, the inductor current may reach zero or

reverse 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 LTC3448 will automatically skip pulses to

maintain output regulation.

Low Ripple LDO Mode Operation

At load currents below ILDO(ON), and when enabled, the

LTC3448 will switch into very low ripple, linear regulating

operation (LRO). In this mode, the current is sourced from

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