LTC4449 datasheet(13/38 Pages) LINER

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LTC3774

3774fc

For more information www.linear.com/LTC3774

OPERATION

Multiphase Operation

For output loads that demand high current, multiple

LTC3774scanbedaisychainedtorunoutofphasetoprovide

more output current without increasing input and output

voltageripple.TheMODE/PLLINpinallowstheLTC3774to

synchronizetotheCLKOUTsignalofanotherLTC3774. The

CLKOUTsignalcanbeconnectedtotheMODE/PLLINpinof

the following LTC3774 stage to line up both the frequency

andthephaseoftheentiresystem.TyingthePHSMDpinto

INTVCC, GND or floating it generates a phase difference

(between CH1 and CLKOUT) of 240°, 60° or 90° respec-

tively, and a phase difference (between CH1 and CH2) of

120°, 180° or 180°. Tying PHSMD to 1/4 or 3/4 of INTVCC

generates a phase difference of 60° and 45° between CH1

and CLKOUT. Figure 1 shows the PHSMD connections

necessary for 3-, 4-, 6-, 8- or 12-phase operation. A total

of 12 phases can be daisychained to run simultaneously

out of phase with respect to each other.

Sensing the Output Voltage with a

Differential Amplifier

The LTC3774 includes a low offset, high input impedance,

unity-gain, high bandwidth differential amplifier for ap-

plications that require true remote sensing. Sensing the

load across the load capacitors directly benefits regulation

in high current, low voltage applications, where board

interconnection losses can be a significant portion of the

total error budget. Connect VOSNS+ to the center tap of the

feedback divider across the output load, and VOSNS– to

the load ground. See Figure 2.

The LTC3774 differential amplifier is configured for unity

gain, meaning that the difference between VOSNS+ and

VOSNS– is translated to its output, relative to GND. The

differential amplifier’s output is internally connected to

the error amplifier inverting input.

Care should be taken to route the VOSNS+ and VOSNS– PCB

traces parallel to each other all the way to the remote sens-

ing points on the board. In addition, avoid routing these

sensitive traces near any high speed switching nodes in

the circuit. Ideally, the VOSNS+ and VOSNS– traces should

be shielded by a low impedance ground plane to maintain

signal integrity.

Figure 2. Differential Amplifier Connection

LTC3774

VOSNS+

CF1

RD1

COUT1

COUT2

VOUT

RD2

VOSNS–

3774 F02

–

DIFFAMP

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