LTC3549 datasheet(8/16 Pages) LINER

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LTC3549

3549f

OPERATIO

Main Control Loop

The LTC3549 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 oscil-

lator 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. The VFB pin, described in

the Pin Functions 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 voltage relative to the 0.611V reference,

which in turn, causes the EA ampliﬁer’s 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.

Comparator OVDET guards against transient overshoots

>10% by turning the main switch off and keeping it off

until the transient has ended.

Burst Mode Operation

The LTC3549 is capable of Burst Mode operation in which

the internal power MOSFETs operate intermittently based

on load demand. To enable Burst Mode operation, simply

connect the MODE pin to GND. To disable Burst Mode

operation and enable PWM pulse-skipping mode, connect

the MODE pin to VIN or drive it with a logic high (VMODE >

1.1V). In this mode, the efﬁciency is lower at light loads,

but becomes comparable to Burst Mode operation when

the output load exceeds 50mA. The advantage of pulse-

skipping mode is lower output ripple and less interference

to audio circuitry. When the converter is in Burst Mode

operation, the minimum peak current of the inductor is

set to approximately 100mA regardless of the output load.

Each burst event can last from a few cycles at light loads

to almost continuously cycling with short sleep intervals

at moderate loads. In between these burst events, the

power MOSFETs and any unneeded circuitry are turned

off, reducing the quiescent current to 50µA. In this sleep

state, the load current is being supplied solely from the

output capacitor. As the output voltage droops, the EA

ampliﬁer’s output rises above the sleep threshold signaling

the BURST comparator to trip and turn the top MOSFET

on. This process repeats at a rate that is dependent on

the load demand.

Short-Circuit Protection

When the output is shorted to ground the LTC3549 limits the

synchronousswitchcurrentto0.45A.Ifthislimitisexceeded,

the top power MOSFET is inhibited from turning on until

the current in the synchronous switch falls below 0.45A.

Dropout Operation

As the input supply voltage decreases to a value approach-

ing the output voltage, the duty cycle increases toward the

maximum on-time. Further reduction of the supply voltage

forces the main switch to remain on for more than one cycle

until it reaches 100% duty cycle. The output voltage will

then be determined by the input voltage minus the voltage

drop across the P-channel MOSFET and the inductor.

Another important detail to remember is that at low input

supply voltages, the RDS(ON) of the P-channel switch

increases (see Typical Performance Characteristics).

Therefore, the user should calculate the power dissipation

when the LTC3549 is used at 100% duty cycle with low

input voltage (see Thermal Considerations in the Applica-

tions Information section).

Slope Compensation

Slope compensation provides stability in constant-fre-

quency architectures by preventing subharmonic oscil-

lations at high duty cycles. It is accomplished internally

by adding a compensating ramp to the inductor current

signal at duty cycles in excess of 40%.

Internal Soft-Start

At start-up when the RUN pin is brought high, the internal

reference is linearly ramped from 0V to 0.611V in 1ms. The

regulated feedback voltage will follow this ramp, resulting

in the output voltage ramping from 0% to 100% in 1ms.

The average current in the inductor during soft-start will

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