LTM4633 datasheet(16/36 Pages) AD

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LTM4671

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

a function of stability and transient response. The Analog

DevicesLTpowerCAD®DesignToolisavailabletodownload

online for output ripple, stability and transient response

analysis and calculating the output ripple reduction as

the number of phases implemented increases by N times.

FORCED CONTINUOUS CURRENT MODE (CCM)

In applications where fixed frequency operation is more

critical than low current efficiency, and where the lowest

outputrippleisdesired,forcedcontinuousoperationshould

beused.Inthismode,inductorcurrentisallowedtoreverse

during low output loads, the COMP voltage is in control

of the current comparator threshold throughout, and the

top MOSFET always turns on with each oscillator pulse.

For the 12A channels (CH0, CH3), CCM can be enabled

by tying the MODE/CLKIN0 or MODE/CLKIN3 pin to the

respective INTVCC or simply floating it.

For the 5A channels (CH1, CH2), CCM can be enabled by

tying the MODE/CLKIN12 pin to GND.

During start-up, forced continuous mode is disabled and

inductor current is prevented from reversing until the

LTM4671’s output voltage is in regulation.

DISCONTINUOUS MODE/BURST MODE OPERATION

Inapplicationswherehighefficiencyatintermediatecurrent

is desired, discontinuous mode or Burst Mode operation

can be achieved.

For the 12A channels (CH0, CH3), discontinuous mode

(DCM) can be achieved by tying the MODE/CLKIN0 or

MODE/CLKIN3 pin to GND. In discontinuous mode, the

reversecurrentcomparatorwillsensetheinductorcurrent

and turn of bottom MOSFET when the inductor current

drops to zero and becomes negative. Both power MOS-

FETs will remain off with the output capacitor supplying

the load current until the COMP voltage rises above its

zero current threshold to initiate the next switching cycle.

For the 5A channels (CH1, CH2), Burst Mode operation

can be achieved by tying MODE/CLKIN12 pin to INTVCC12

or simply floating. In Burst Mode operation, a current

reversal comparator (IREV) detects the negative inductor

current and shuts off the bottom power MOSFET, resulting

in discontinuous operation and increased efficiency. Both

power MOSFETs will remain off until the ITH voltage rises

abovethezerocurrentleveltoinitiateanothercycle.During

this time, the output capacitor supplies the load current

and the part is placed into a low current sleep mode.

OPERATING FREQUENCY

The operating frequency of the LTM4671 is optimized

to achieve the compact package size and the minimum

output ripple voltage while still keeping high efficiency.

The default operating frequency is internally set to 600kHz

for 12A channels and 1MHz for 5A channels. In most ap-

plications, no additional frequency adjusting is required.

Forthe12Achannels(CH0,CH3),ifanoperatingfrequency

other than 600kHz is required by the application, the op-

erating frequency can be increased by adding a resistor,

RFSET, between the FREQ0 or FREQ3 pins and SGND. The

operating frequency can be calculated as:

f Hz

( ) =

1.6e11

274k ||RFSET Ω

( )

The programmable operating frequency range is from

400kHz to 3MHz.

For the 5A channels (CH1, CH2), If an operating frequency

other than 1MHz is required by the application, the op-

erating frequency can be increased by adding a resistor,

RFSET, between the FREQ12 pin and SGND. The operating

frequency can be calculated as:

f Hz

( ) =

3.2e11

324k ||RFSET Ω

( )

The programmable operating frequency range is from

400kHz to 3MHz.

AlsotheµModulecanbeexternallysynchronizedtoaclock

at ±30% around set operating frequency.

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