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LTC3874-1

38741f

For more information www.linear.com/LTC3874-1

operaTion

Main Control Loop

The LTC3874-1 is a constant frequency, LTC proprietary

currentmodestep-downslavecontrollerforparallelopera-

tionwithmastercontrollers.Duringnormaloperation,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 ITHpin,whichistheoutputofthemaster

controller. When the load current increases, the master

controller increases the ITH voltage, which in turn causes

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 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

ConductionMode(DCM).TheLTC3874-1slavecontrollers

DO NOT regulate the output voltage but regulate the cur-

rent in each channel for current sharing with the master

controllers. Output voltage regulation is achieved through

thevoltagefeedbackcontrolloopinthemastercontrollers.

Sub-Milliohm DCR Current Sensing

The LTC3874-1 employs a unique architecture to enhance

the signal-to-noise ratio that enables it to operate with

a small sense signal of a sub-milliohm value inductor

DCR to improve power efficiency and reduce jitter due to

switching noise.

Floating or pulling the LOWDCR pin high will enable sub-

milliohm DCR current sensing. The LTC3874-1 can sense

a DCR value as low as 0.2mΩ with careful PCB layout.

The proprietary signal processing circuit provides a 14dB

signal-to-noise ratio improvement. As with conventional

current mode architectures, the current limit threshold is

still a function of the inductor peak current and the DCR

value, and can be accurately set with the ILIM and ITH pins.

INTVCC/EXTVCC Power

PowerformostinternalcircuitryisderivedfromtheINTVCC

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

less than 4.7V, an internal 5.5V linear regulator supplies

INTVCC power from VIN. If EXTVCC is taken above 4.7V

and VIN is higher than 7V, the 5.5V regulator is turned off

and an internal switch is turned on connecting EXTVCC.

EXTVCCcanbeappliedbeforeVIN.UsingtheEXTVCCallows

the INTVCC power to be drawn from an external source.

Start-Up and Shutdown (RUN0, RUN1)

The two channels of the LTC3874-1 can be independently

shut down using the RUN0 and RUN1 pins. Pulling either

of these pins below 1.4V shuts down the main control

circuits for that channel. During shutdown, the PWM pin is

in three-state mode. Pulling either of these pins above 2V

enables the controller. The RUN0/1 pins are actively pulled

down until the INTVCC voltage passes the undervoltage

lockout threshold of 3.8V. For multiphase operation, the

RUN0/1 pins must be connected together and driven by

the RUN pins on the master controller. Because a large

RC filter in the LTC3874-1 needs to settle during initializa-

tion, the RUN pins can only be pulled up 4ms after VIN

is ready. Do not exceed the Absolute Maximum Rating of

6V on these pins.

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

controlled by the master controller. After the RUN pins are

released, the master controller drives the output based on

the programmed delay time and rise time. The slave con-

troller LTC3874-1 follows the ITHvoltagesetbythemaster

to supply the same current to the output during startup.

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