LTM4627_15 datasheet(13/32 Pages) LINER

13 / 32 page

LTM4627

4627fc

For more information www.linear.com/LTM4627

Multiphase Operation

For outputs that demand more than 15A of load current,

multiple LTM4627 devices can be paralleled to provide

more output current without increasing input and output

ripple voltage. The MODE_PLLIN pin allows the LTM4627

to be synchronized to an external clock and the internal

phase-locked loop allows the LTM4627 to lock onto input

clock phase as well. The fSET resistor is selected for nor-

mal frequency, then the incoming clock can synchronize

the device over the specified range. See Figure 20 for a

synchronizing example circuit.

A multiphase power supply significantly reduces the

amount of ripple current in both the input and output ca-

pacitors. The RMS input ripple current is reduced by, and

the effective ripple frequency is multiplied by, the number

of phases used (assuming that the input voltage is greater

than the number of phases used times the output voltage).

The output ripple amplitude is also reduced by the number

of phases used. See Application Note 77.

The LTM4627 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

the COMP and VFB pins of each LTM4627 together to

share the current evenly. Figure 20 shows a schematic of

the parallel design.

Input RMS Ripple Current Cancellation

Application Note 77 provides a detailed explanation of

multiphase operation. The input RMS ripple current can-

cellation mathematical derivations are presented, and a

graph is displayed representing the RMS ripple current

reductionasafunctionofthenumberofinterleavedphases

(see Figure 2).

PLL, Frequency Adjustment and Synchronization

TheLTM4627switchingfrequencyissetbyaresistor(RfSET)

from the fSET pin to signal ground. A 10µA current (IFREQ)

flowing out of the fSET pin through RfSET develops a volt-

age on fSET. RfSET can be calculated as:

RfSET =

FREQ

500kHz / V

+0.2V









10µA

The relationship of fSET voltage to switching frequency is

shown in Figure 3. For low output voltages from 0.8V to

1.5V,400kHzoperationisanoptimalfrequencyforthebest

power conversion efficiency while maintaining the induc-

tor ripple current to about 30% to 40% of maximum load

current. For output voltages from 1.8V to 3.0V, 500kHz to

600kHz is optimal. For output voltages from 3.0V to 5.0V,

750kHz operation is optimal, but due to the higher ripple

currentat5Voperationtheoutputcurrentislimitedto10A.

TheLTM4627canbesynchronizedfrom250kHzto800kHz

with an input clock that has a high level above 2V and a

low level below 0.8V. However, a 400kHz low end operating

frequencyisrecommendedtolimitinductorripplecurrent.

See the Typical Applications section for synchronization

examples. The LTM4627 minimum on-time is limited to

approximately 90ns. Guardband the on-time to 130ns.

The on-time can be calculated as:

tON(MIN)=

FREQ

•

VOUT

VIN













Output Voltage Tracking

Output voltage tracking can be programmed externally

using the TRACK/SS pin. The output can be tracked up

and down with another regulator. The master regulator’s

output is divided down with an external resistor divider

that is the same as the slave regulator’s feedback divider

to implement coincident tracking. The LTM4627 uses an

accurate 60.4k resistor internally for the top feedback

resistor.Figure4showsanexampleofcoincidenttracking.

VOUT(SLAVE) = 1+

60.4k

RTA











 • VTRACK

VTRACK is the track ramp applied to the slave’s track pin.

VTRACK has a control range of 0V to 0.6V, or the internal

reference voltage. When the master’s output is divided

down with the same resistor values used to set the slave’s

output, then the slave will coincident track with the master

until it reaches its final value. The master will continue to

APPLICATIONS INFORMATION

LTM4627 Datasheet(PDF) 13 Page - Linear Technology

LTM4627 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL