LTM4633 datasheet(12/32 Pages) LINER

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LTM4633

4633f

For more information www.linear.com/LTM4633

applicaTions inForMaTion

above formula. This will give the maximum RMS capacitor

current requirement. Increasing the output current drawn

from the other channels will actually decrease the input

RMS ripple current from its maximum value. The out-of-

phase technique typically reduces the input capacitor’s

RMS ripple current by a factor of 50% when compared to

a single phase power supply solution. If the three channels

are powered from independent input sources, then each

of the input RMS current ratings will need to be calculated

specific to that channel.

Output Capacitors

The LTM4633 is designed for low output voltage ripple

noise. The bulk output capacitors defined as COUT are

chosen with low enough effective series resistance (ESR)

to meet the output voltage ripple and transient require-

ments. COUTcanbealowESRtantalumcapacitor,lowESR

Polymer capacitor or ceramic capacitor. The typical output

capacitancerangeisfrom200µFto470µF.Additionaloutput

filtering may be required by the system designer if further

reduction of output ripple or dynamic transient spikes is

required.Table5showsamatrixofdifferentoutputvoltages

and output capacitors to minimize the voltage droop and

overshoot during a 5A/µs transient. The table optimizes

total equivalent ESR and total bulk capacitance to optimize

thetransientperformance.Stabilitycriteriaareconsidered

in the Table 5 matrix, and LTpowerCAD is available for free

to conduct stability analysis. LTpowerCAD can calculate

the output ripple reduction as the number of implemented

phases increases by N times.

Burst Mode Operation

The LTM4633 is capable of Burst Mode operation in

which the power MOSFETs operate intermittently based

on load demand, thus saving quiescent current. For ap-

plications where maximizing the efficiency at very light

loads is a high priority, Burst Mode operation should be

applied. To enable Burst Mode operation, simply float

the MODE/PLLIN pin. During Burst Mode operation, the

peak current of the inductor is set to approximately 30%

of the maximum peak current value in normal operation

even though the voltage at the COMP pin indicates a

lower value. The voltage at the COMP pin drops when

the inductor’s average current is greater than the load

requirement. As the COMP voltage drops below 0.5V, the

burst comparator trips, causing the internal sleep line to

go high and turn off both power MOSFETs.

In sleep mode, the internal circuitry is partially turned

off, reducing the quiescent current. The load current is

now being supplied from the output capacitors. When the

output voltage drops, causing COMP to rise, the internal

sleep line goes low, and the LTM4633 resumes normal

operation. The next oscillator cycle will turn on the top

power MOSFET and the switching cycle repeats.

Pulse-Skipping Mode Operation

Inapplicationswherelowoutputrippleandhighefficiency

at intermediate currents are desired, pulse-skipping

mode should be used. Pulse-skipping operation allows

the LTM4633 to skip cycles at low output loads, thus

increasing efficiency by reducing switching loss. Tying

the MODE/PLLIN pin to INTVCC enables pulse-skipping

operation. With pulse-skipping mode at light load, the

internalcurrentcomparatormayremaintrippedforseveral

cycles, thus skipping operation cycles. This mode has

lower ripple than Burst Mode operation and maintains a

higher frequency operation than Burst Mode operation.

Forced Continuous Operation

In applications where fixed frequency operation is more

critical than low current efficiency, and where the lowest

output ripple is desired, forced continuous operation

should be used. Forced continuous operation can be

enabled by tying the MODE/PLLIN pin to ground. In this

mode, inductor current is allowed to reverse during low

output loads, the COMP voltage is in control of the current

comparator threshold throughout, and the top MOSFET

alwaysturnsonwitheachoscillatorpulse.Duringstart-up,

forced continuous mode is disabled and inductor current

is prevented from reversing until the LTM4633 output

voltage is in regulation.

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