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LTC3870

3870fb

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operaTion

Main Control Loop

The LTC3870 is a constant frequency, current mode

step-down slave controller for parallel operation with the

LTC3880 family master controllers. During normal opera-

tion, 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 tied

directly to the corresponding ITHpinofthemastercontrol-

lers. When the load current increases, master controllers

drive and increase the ITH voltage, which in turn cause

the peak current in the corresponding slave channels to

increase, until the average inductor current matches the

new load current. After the top MOSFET has been turned

off, the bottom MOSFET is turned on until the beginning

of the next cycle in Continuous Conduction Mode (CCM)

or until the inductor current starts to reverse, as indicated

by the reverse current comparator IREV, in Discontinuous

Conduction Mode (DCM). The LTC3870 slave controllers

DONOTregulatetheoutputvoltagebutregulatethecurrent

ineachchannelforcurrentsharingwithmastercontrollers.

Output voltage regulation is achieved through the voltage

feedback loops in master controllers.

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

other internal circuitry is derived from the INTVCC pin.

Normally an internal 5.0V linear regulator supplies INTVCC

power from VIN. In high VIN applications, if a high effi-

ciency external voltage source is available for the EXTVCC

pin, another internal 5.0V linear regulator is enabled and

supplies INTVCC power from EXTVCC. To enable the linear

regulator driven by the EXTVCC pin, VIN has to be higher

than 6.5V and EXTVCC pin voltage has to be higher than

4.8V. Do not exceed 14V on the EXTVCC pin.

Each top MOSFET driver is biased from the floating

bootstrap capacitor CB, which normally recharges during

each 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 detector detects this and forces the top MOSFET

off for about one-twelfth of the clock period plus 100ns

every three cycles to allow CB to recharge. However, it is

recommended that a load be present or the IC operates

at low frequency during the drop-out transition to ensure

CB is recharged.

Start-Up and Shutdown (RUN0, RUN1)

The two channels of the LTC3870 can independently

start up and shut down using the RUN0 and RUN1 pins.

Pulling either of these pins below 1.4V shuts down the

control circuits for that channel. During shutdown, both

TG and BG are pulled down to turn off the external power

MOSFETs. Pulling either of these pins above 2V enables

the corresponding channel and internal circuits. During

startup, the RUN0/RUN1 pins are actively pulled down

until the INTVCC voltage passes the under-voltage lockout

threshold of 4V. For multiphase parallel operation, the

RUN0/RUN1 pins have to be connected and driven by

the RUN pins of the master controller. Do not exceed the

Absolute Maximum Rating of 6V on these pins.

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

controlledandprogrammedbythemastercontroller.After

the RUN pins are released, the master controller drives the

output based on the programmed delay time and rise time,

and the slave controller LTC3870 just follows the master

to supply equivalent current to the output during startup.

Light Load Current Operation (Discontinuous

Conduction Mode, Continuous Conduction Mode)

The LTC3870 can be set to operate either in Discontinuous

Conduction Mode (DCM) or forced Continuous Conduc-

tion Mode (CCM). To select forced Continuous Mode of

operation, tie the MODE pin to a DC voltage above 2V

(e.g., INTVCC). To select discontinuous conduction mode

of operation, tie the MODE pin to a DC voltage below 1.4V

(e.g., SGND). In forced continuous operation, the induc-

tor current is allowed to reverse at light loads or under

large transient conditions. The peak inductor current is

determined by the voltage on the ITH pin. In this mode,

the efficiency at light loads is lower than in discontinu-

ous Mode operation. However, continuous mode has the

advantages of lower output ripple and less interference

with audio circuitry. When the MODE pin is connected to

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