LTC3853_15 datasheet(11/36 Pages) LINER

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LTC3853

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OPERATION

Main Control Loop

The LTC3853 is a constant-frequency, current mode

step-down controller with three channels operating 120

degrees out-of-phase. During normal operation, each

top MOSFET is turned on when the clock for that channel

sets the RS latch, and turned off when the main current

comparator, ICMP, resets the RS latch. The peak inductor

current at which ICMP resets the RS latch is controlled by

the voltage on the ITH pin, which is the output of each error

amplifier, EA. The VFB pin receives the voltage feedback

signal, which is compared to the internal reference voltage

by the EA. When the load current increases, it causes a

slight decrease in VFB relative to the 0.8V reference, which

in turn causes the ITH voltage to increase until the average

inductor current matches the new load current. After the

top MOSFET has turned off, the bottom MOSFET is turned

on until either the inductor current starts to reverse, as

indicated by the reverse current comparator IREV, or the

beginning of the next cycle.

INTVCC/EXTVCC/DRVCC12 Power

Power for the top and bottom MOSFET drivers of phase 3

and most other internal circuitry is derived from the

INTVCC pin. DRVCC12 provides driver power for phase 1

and phase 2. This pin must be externally tied to INTVCC.

If EXTVCC is taken above 4.7V, the 5V regulator is turned

off and an internal switch is turned on connecting EXTVCC.

Using the EXTVCC pin allows the INTVCC power to be de-

rived from a high efficiency external source such as one

of the LTC3853 switching regulator outputs.

Each top MOSFET driver is biased from the floating boot-

strap capacitor, CB,whichnormallyrechargesduringeach

off cycle through an external diode when the top MOSFET

turns off. If the input voltage, VIN, decreases to a voltage

close to VOUT, the loop may enter dropout and attempt to

turn on the top MOSFET continuously. The dropout detec-

tor detects this and forces the top MOSFET off for about

one-twelfth of the clock period every fifth cycle to allow

CB to recharge. However, it is recommended that there is

always a load be present during the dropout transition to

ensure CB is recharged.

Shutdown and Start-Up (RUN1, RUN2, RUN3 and TK/

SS1, TK/SS2, TK/SS3 Pins)

The three channels of the LTC3853 can be independently

shut down using the RUN1, RUN2 and RUN3 pins. Pulling

any of these pins below 1.2V shuts down the main control

loop for that controller. Pulling all pins low disables all

three controllers and most internal circuits, including the

INTVCCregulator.ReleasinganyRUNpinallowsaninternal

0.5µA current to pull up the pin and enable that controller.

Alternatively, the RUN pin may be externally pulled up

or driven directly by logic. Be careful not to exceed the

absolute maximum rating of 6V on this pin.

The start-up of each controller’s output voltage VOUT is

controlled by the voltage on the TK/SS1, TK/SS2 and

TK/SS3 pins. When the voltage on the TK/SS pin is less

than the 0.8V internal reference, the LTC3853 regulates

the VFB voltage to the TK/SS pin voltage instead of the

0.8V reference. This allows the TK/SS pin to be used to

program a soft-start by connecting an external capacitor

from the TK/SS pin to SGND. An internal 1.3µA pull-up

current charges this capacitor, creating a voltage ramp on

the TK/SS pin. As the TK/SS voltage rises linearly from

0V to 0.8V (and beyond), the output voltage VOUT rises

smoothly from zero to its final value. Alternatively the TK/

SS pin can be used to cause the start-up of VOUT to “track”

that of another supply. Typically, this requires connect-

ing to the TK/SS pin an external resistor divider from the

other supply to ground (see the Applications Information

section). When the corresponding RUN pin is pulled low

to disable a controller, or when INTVCC drops below its

undervoltage lockout threshold of 3.35V, the TK/SS pin

is pulled low by an internal MOSFET. When in undervolt-

age lockout, all controllers are disabled and the external

MOSFETs are held off.

Light Load Current Operation (Burst Mode Operation,

Pulse Skipping or Continuous Conduction)

The LTC3853 can be enabled to enter high efficiency Burst

Modeoperation,constant-frequencypulseskippingmode,

or forced continuous conduction mode. To select forced

continuous operation, tie the MODE/PLLIN pin to a DC

voltage below 0.8V (e.g., SGND). To select pulse skipping

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