LTM4677 datasheet(9/22 Pages) LINER

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LTC3874-1

38741f

For more information www.linear/LTC3874-1

operaTion

Light Load Current Operation (Discontinuous

Conduction Mode, Continuous Conduction Mode)

The LTC3874-1 can operate either in discontinuous con-

duction mode or forced continuous conduction mode.

To select forced continuous mode, tie the MODE pin to a

DC voltage above 2V (e.g., INTVCC). To select discontinu-

ous conduction mode, tie the MODE pin to a DC voltage

below 1.4V (e.g., GND).In forced continuous mode,

the inductor 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

discontinuous mode. However, continuous mode has the

advantages of lower output ripple and less interference

with audio circuitry. When the MODE pin is connected to

GND, the LTC3874-1 operates in discontinuous mode at

light loads. At very light loads, the current comparator

ICMP may remain tripped for several cycles and force the

external top MOSFET to stay off for the same number of

cycles (i.e., skipping pulses). This mode provides higher

light load efficiency than forced continuous mode and the

inductor current is not allowed to reverse. There is a 500k

pull-down resistor internally connected to the MODE pin.

If the MODE0/1 pins are left floating, both channels are in

discontinuous conduction mode by default.

Multichip Operations (PHASMD and SYNC Pins)

The PHASMD pin determines the relative phases between

the internal channels as well as the external clock signal on

SYNC pin as shown in Table 1. The phases tabulated are

relative to zero degree phase being defined as the falling

edge of the clock on SYNC pin.

Table 1

PHASMD

CHANNEL 0 PHASE

CHANNEL 1 PHASE

GND

180°

0°

1/3 INTVCC

60°

300°

2/3 INTVCC or Float

120°

240°

INTVCC

90°

270°

The SYNC pin is used to synchronize switching frequency

betweenthemasterandslavecontrollers.Inputcapacitance

ESR requirements and efficiency losses are substantially

reduced because the peak current drawn from the input

capacitor is effectively divided by the number of phases

used and power loss is proportional to the RMS current

squared. A two stage, single output voltage implementa-

tion can reduce input path power loss by 75% and radi-

cally reduce the required RMS current rating of the input

capacitor(s).

Single Output Multiphase Operation

The LTC3874-1 is configured for single output multiphase

converters with a master controller by making these con-

nections

• Tie all the ITH pins of paralleled channels together for

current sharing between masters and slaves;

• Tie all SYNC or PLLIN pins of paralleled channels to-

gether or tie the master chip’s CLKOUT pin to the slave

chip’sSYNCpinforswitchingfrequencysynchronization

among channels.

• Tie all the RUN pins of paralleled channels together for

startup and shutdown at the same time.

• Tie the fault indictor pin of the master controller if avail-

able to the FAULT pin of the slave controller for fault

protection.

• The LTC3874-1 MODE pin can be tied to the master chip

PGOOD pin for start-up control. During soft-start, the

LTC3874-1 operates in DCM mode. When the soft-start

interval is done, the LTC3874-1 operates in CCM mode.

Examples of single output multiphase converters are

shown in Figure 1.

The Typical Application on the first page of this data sheet

is a basic LTC3874-1 application circuit configured as a

slavecontroller.Inparalleledoperation,thecurrentsensing

scheme and circuit parameters in the LTC3874-1 have to

be the same as the master controller to achieve balanced

current sharing between masters and slaves. Input and

output capacitors are selected based on RMS current

rating, ripple and transient specs.

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