LTC3859 datasheet(14/40 Pages) LINER

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LTC3859

3859f

OPERATION

Main Control Loop

The LTC3859 uses a constant frequency, current mode

step-down architecture. The two buck controllers, chan-

nels 1 and 2, operate 180 degrees out of phase with each

other. The boost controller, channel 3, operates in phase

with channel 1. During normal operation, the external

top MOSFET for the buck channels (the external bottom

MOSFET for the boost channel) is turned on when the

clock for that channel sets the RS latch, and is turned off

when the main current comparator, ICMP, resets the RS

latch. The peak inductor current at which ICMP trips and

resets the latch is controlled by the voltage on the ITH pin,

which is the output of the error ampliﬁer EA. The error

ampliﬁer compares the output voltage feedback signal at

the VFB pin, (which is generated with an external resistor

divider connected across the output voltage, VOUT, to

ground) to the internal 0.800V reference voltage for the

bucks (1.2V reference voltage for the boost). When the

load current increases, it causes a slight decrease in VFB

relative to the reference, which causes the EA to increase

the ITH voltage until the average inductor current matches

the new load current.

After the top MOSFET for the bucks (the bottom MOSFET

for the boost) is turned off each cycle, the bottom MOSFET

is turned on (the top MOSFET for the boost) until either

the inductor current starts to reverse, as indicated by the

current comparator IR, or the beginning of the next clock

cycle.

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTVCC pin.

When the EXTVCC pin is left open or tied to a voltage less

than 4.7V, the VBIAS LDO (low dropout linear regulator)

supplies 5.4V from VBIAS to INTVCC. If EXTVCC is taken

above 4.7V, the VBIAS LDO is turned off and an EXTVCC

LDO is turned on. Once enabled, the EXTVCC LDO supplies

5.4V from EXTVCC to INTVCC. Using the EXTVCC pin allows

the INTVCC power to be derived from a high efﬁciency

external source such as one of the LTC3859 switching

regulator outputs.

Each top MOSFET driver is biased from the ﬂoating boot-

strap capacitor CB, which normally recharges during each

cycle through an external diode when the switch voltage

goes low.

For buck channels 1 and 2, if the buck’s input voltage

decreases to a voltage close to its output, the loop may

enter dropout and attempt to turn on the top MOSFET

continuously. The dropout detector detects this and forces

the top MOSFET off for about one twelfth of the clock

period every tenth cycle to allow CB to recharge.

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

TRACK/SS1, TRACK/SS2, SS3 Pins)

The three channels of the LTC3859 can be independently

shut down using the RUN1, RUN2 and RUN3 pins. Pull-

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

control loop for that channel. Pulling all three pins below

0.7V disables all controllers and most internal circuits,

including the INTVCC LDOs. In this state, the LTC3859

draws only 14μA of quiescent current.

Releasing a RUN pin allows a small internal current to pull

up the pin to enable that controller. The RUN1 pin has a

6μA pull-up current while the RUN2 and RUN3 pins have

a smaller 0.5μA. The 6μA current on RUN1 is designed

to be large enough so that the RUN1 pin can be safely

ﬂoated (to always enable the controller) without worry

of condensation or other small board leakage pulling the

pin down. This is ideal for always-on applications where

one or more controllers are enabled continuously and

never shut down.

Each RUN pin may also be externally pulled up or driven

directly by logic. When driving a RUN pin with a low im-

pedance source, do not exceed the absolute maximum

rating of 8V. Each RUN pin has an internal 11V voltage

clamp that allows the RUN pin to be connected through

a resistor to a higher voltage (for example, VBIAS), so

long as the maximum current in the RUN pin does not

exceed 100μA.

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

trolled by the voltage on the TRACK/SS pin (TRACK/SS1 for

channel 1, TRACK/SS2 for channel 2, SS3 for channel 3).

When the voltage on the TRACK/SS pin is less than the

(Refer to Functional Diagram)

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