LTM4644 datasheet(12/32 Pages) LINER

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LTM4634

4634f

For more information www.linear.com/LTM4634

applicaTions inForMaTion

of the input RMS current ratings will need to be calculated

specific to that channel.

Output Capacitors

The LTM4634 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. COUT can be a low ESR tantalum capacitor, low

ESR Polymer capacitor or ceramic capacitor. The typical

outputcapacitancerangeisfrom200µFto470µF.Additional

output filtering may be required by the system designer

if further reduction of output ripple or dynamic transient

spikesisrequired.Table4showsamatrixofdifferentoutput

voltages 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 the transient performance. Stability criteria

are considered in the Table 4 matrix, and LTpowerCAD is

available for stability analysis. LTpowerCAD can calculate

the output ripple reduction as the number of implemented

phases increases by N times.

Burst Mode Operation

The LTM4634 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 LTM4634 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 LTM4634 to skip cycles at low output loads, thus

increasing efficiency by reducing switching loss. Tying

the MODE/PLLINpin 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 LTM4634 output

voltage is in regulation.

Frequency Synchronization

The LTM4634 device operates up to 750kHz. It can also be

synchronizedwithaninputclockthathasahighlevelabove

2V and a low level below 0.8V at the MODE/PLLIN pin. The

FREQ/PLLLPF pin must be floating when synchronized to

an incoming clock. Once the LTM4634 is synchronized to

an external clock frequency, it will always be running in

forced continuous operation. The synchronizing range is

from 250kHz to 750kHz. For VOUT1,2,3 ≤ 1.5V use 250kHz

to 300kHz, 1.5V ≤ VOUT1,2,3 ≤ 2.5V use 400kHz, 2.5V ≤

VOUT1,2,3 ≤ 5V use 600kHz. If VOUT3 is greater than 5V

up to 12V set the operating frequency to 750kHz. These

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