LTC3883 datasheet(10/22 Pages) LINER

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LTC3870

3870fb

For more information www.linear.com/LTC3870

SGND, the LTC3870 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 are 500k

pull down resistors internally connected to the MODE0/

MODE1 pins. If MODE0/MODE1 pins are floating, both

channels default to discontinuous conduction mode.

Multichip Operation (PHASMD and SYNC Pins)

The PHASMD pin determines the relative phases between

the internal channels as well as the external clock signal on

the 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.

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

between master and slave controllers. Input capacitance

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-phase, 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).

operaTion

Single Output Multiphase Operation

The LTC3870 is designed for multiphase converters with

the master controller by making these connections:

Tie all the ITH pins of paralleled channels together for

current sharing between masters and slaves. Note that

ILIM setup on slaves has to match MFR_PWM_MODE

current range setup in masters.

Tie all SYNC pins together between master and slaves for

sameswitchingfrequencysynchronization;oneandonly

one of the master controllers has to be programmed

as master to generate clock signal on the SYNC pin.

TiealltheRUNpinsofparalleledchannelstogetherbetween

masterandslavesforstartupandshutdownsequences.

Tie the GPIO pin of the master controller to the FAULT pin

of slave controllers and program the master GPIO as

fault sharing for fault protections.

Examples of single output multiphase converters are

shown in Figure 1.

Inductor Current Sensing

LiketheLTC3880/LTC3883,LTC3870canuseeitherinduc-

tor DCR or RSENSE to sense the inductor current. Inductor

DCR current sensing provides a lossless method of sens-

ing the instantaneous current. Therefore, it can provide

higherefficiencyforapplicationswithhighoutputcurrents.

However, the DCR of a copper inductor typically has 10%

tolerance.Forprecisecurrentsensing,aprecisionsensing

resistor RSENSE can be used to sense the inductor current.

Itisimportanttomatchthecurrentsensingcircuitbetween

master controllers and slave controllers to guarantee bal-

anced load sharing and overcurrent protection.

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