LTM4613 datasheet(11/28 Pages) LINER

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LTM4648

4648f

For more information www.linear.com/LTM4648

applicaTions inForMaTion

output filtering may be required by the system designer

if further reduction of output ripple or dynamic transient

spikes is required. Table 3 shows a matrix of different

output 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. Stabil-

ity criteria are considered in the Table 3 matrix, and the

Linear Technology µModule Power Design Tool will be

provided for stability analysis. Multiphase operation will

reduce effective output ripple as a function of the num-

ber of phases. Application Note 77 discusses this noise

reduction versus output ripple current cancellation, but

the output capacitance should be considered carefully as

a function of stability and transient response. The Linear

Technology µModule Power Design Tool can calculate the

output ripple reduction as the number of implemented

phase’s increases by N times.

Burst Mode Operation

The LTM4648 is capable of Burst Mode operation in which

the power MOSFETs operate intermittently based on load

demand, thus saving quiescent current. For applications

where maximizing the efficiency at very light loads is a

high priority, Burst Mode operation should be applied. To

enable Burst Mode operation, simply tie the MODE pin to

INTVCC. During Burst Mode operation, the peak current

of the inductor is set to approximately 30% of the maxi-

mum 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 aver-

age 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 LTM4648 resumes normal

operation. The next oscillator cycle will turn on the top

power MOSFET and the switching cycle repeats.

Pulse-Skipping Mode Operation

In applications where low output ripple and high efficiency

at intermediate currents are desired, pulse-skipping mode

should be used. Pulse-skipping operation allows the

LTM4648toskipcyclesatlowoutputloads,thusincreasing

efficiency by reducing switching loss. Floating the MODE

pinenablespulse-skippingoperation. Withpulse-skipping

mode at light load, the internal current comparator may

remain tripped for several cycles, thus skipping opera-

tion cycles. This mode has lower ripple than Burst Mode

operationandmaintainsahigherfrequencyoperationthan

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 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 LTM4648’s output

voltage is in regulation.

Frequency Selection

The LTM4648 device is internally programmed to 450kHz

switching frequency to improve power conversion ef-

ficiency. It is recommended for all of the application.

If desired, a resistor can be connected from the FREQ pin

to INTVCC to adjust the FREQ pin DC voltage to increase

the switching frequency between default 450kHz and

maximum 650kHz. Figure 2 shows a graph of frequency

setting verses FREQ pin DC voltage. Figure 18 shows an

example of frequency programmed to 650kHz. Please be

aware FREQ pin has an accurate 10µA current sourced

from this pin when calculate the resistor value.

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