LTM4608 datasheet(11/24 Pages) LINER

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LTM4608

4608f

APPLICATIONS INFORMATION

OUT

Without considering the inductor current ripple, the RMS

current of the input capacitor can be estimated as:

CIN(RMS) =

OUT(MAX)

η%

•D • 1– D

()

In the above equation, η% is the estimated efﬁciency of

the power module. The bulk capacitor can be a switcher-

rated electrolytic aluminum capacitor, polymer capacitor

for bulk input capacitance due to high inductance traces

or leads. If a low inductance plane is used to power the

device, then only one 10μF ceramic is required. The three

internal 10μF ceramics are typically rated for 2A of RMS

ripple current, so the ripple current at the worse case for

8A maximum current is 4A or less.

Output Capacitors

The LTM4608 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, a low

ESR polymer capacitor or ceramic capacitor. The typical

output capacitance range is from 47μF to 220μF. Additional

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 3A/μs transient. The table

optimizes total equivalent ESR and total bulk capacitance

to optimize the transient performance. Stability 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 number of phases. Application

Note 77 discusses this noise reduction versus output

ripple current cancellation, but the output capacitance

will be more a function of stability and transient response.

The Linear Technology μModule Power Design Tool will

calculate the output ripple reduction as the number phases

implemented increases by N times.

Burst Mode Operation

The LTM4608 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

VIN. During this operation, the peak current of the inductor

is set to approximately 20% of the maximum peak current

value in normal operation even though the voltage at the

ITH pin indicates a lower value. The voltage at the ITH pin

drops when the inductor’s average current is greater than

the load requirement. As the ITH voltage drops below 0.2V,

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, re-

ducing the quiescent current to about 450μA. The load cur-

rent is now being supplied from the output capacitor. When

the output voltage drops, causing ITH to rise above 0.25V,

the internal sleep line goes low, and the LTM4608 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

LTM4608 to skip cycles at low output loads, thus increasing

efﬁciency by reducing switching loss. Floating the MODE

pin or tying it to VIN/2 enables pulse-skipping operation.

This allows discontinuous conduction mode (DCM) opera-

tion down to near the limit deﬁned by the chip’s minimum

on-time (about 100ns). Below this output current level,

the converter will begin to skip cycles in order to main-

tain output regulation. Increasing the output load current

slightly, above the minimum required for discontinuous

conduction mode, allows constant frequency PWM.

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

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