LTM4627EVPBF datasheet(11/28 Pages) LINER

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LTM4627

4627f

Output Capacitors

The LTM4627 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 capacitors. The typical output

capacitance range is from 200μF to 800μF. Additional output

ﬁltering may be required by the system designer if further

reduction of output ripple or dynamic transient spikes is

required. Table 4 shows a matrix of different output voltages

and output capacitors to minimize the voltage droop and

overshoot during a 7A/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 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 number 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 LTM4627 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 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 LTM4627 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 efﬁ-

ciency at intermediate currents are desired, pulse-skipping

mode should be used. Pulse-skipping operation allows

the LTM4627 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

internal current comparator may remain tripped for several

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

always turns on with each oscillator pulse. During start-up,

forced continuous mode is disabled and inductor current

is prevented from reversing until the LTM4627’s output

voltage is in regulation.

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