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LTM4648

4648f

For more information www.linear.com/LTM4648

applicaTions inForMaTion

PLL and Frequency Synchronization

The LTM4648 device operates over a range of frequen-

cies to improve power conversion efficiency. The nominal

switchingfrequencyis450kHz.Itcanalsobesynchronized

from 350kHz to 650kHz with an input clock that has a high

level above 2V and a low level below 0.8V at the CLKIN pin.

Once the LTM4648 is synchronizing to an external clock

frequency, it will always running in Forced Continuous

Operation. The 350kHz low end operation frequency limit

is put in place to limit inductor ripple current.

Multiphase Operation

For outputs that demand more than 10A of load current,

multiple LTM4648 devices can be paralleled to provide

more output current and reduced input and output volt-

age ripple.

The CLKOUT signal together with CLKIN pin can be used

to cascade additional power stages to achieve the multi-

phase power supply solution. Tying the PHMODE pin to

INTVCC, GND, or (floating) generates a phase difference

(between MODE/PLLIN and CLKOUT) of 180°, 120°, or

90° respectively as shown in Table 2. A total of 4 phases

can be cascaded to run simultaneously with respect to

each other by programming the PHMODE pin of each

LTM4648 channel to different levels. Figure 3 shows a

3-phase design and 4-phase design example for clock

phasing with the PHASMD table.

Table 2. PHASEMD and CLKOUT Signal Relationship

PHASEMD

GND

FLOAT

INTVCC

CLKOUT

120°

90°

180°

The LTM4648 device is an inherently current mode con-

trolled device, so parallel modules will have good current

sharing. This will balance the thermals in the design. Tie

theCOMP,VFB,TRACK/SSandRUNpinsofeachLTM4648

together to share the current evenly. Figures 20 and 21

show a schematic of the parallel design.

4648 F03

4-PHASE DESIGN

3-PHASE DESIGN

90 DEGREE

0 PHASE

FLOAT

CLKIN

VOUT

PHASMD

CLKOUT

90 PHASE

CLKIN

VOUT

PHASMD

CLKOUT

180 PHASE

CLKIN

VOUT

PHASMD

CLKOUT

270 PHASE

CLKIN

VOUT

PHASMD

CLKOUT

120 DEGREE

0 PHASE

GND

CLKIN

VOUT

PHASMD

CLKOUT

120 PHASE

CLKIN

VOUT

PHASMD

CLKOUT

240 PHASE

CLKIN

VOUT

PHASMD

CLKOUT

Figure 3. Examples of 3-Phase, 4-Phase Operation with PHASMD Table

FREQ PIN VOLTAGE (V)

900

800

600

400

100

200

700

500

300

4648 F02

2.5

1.5

0.5

Figure 2. Operating Frequency vs FREQ Pin Voltage

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