LTM4613 datasheet(14/32 Pages) LINER

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LTM4653

Rev 0

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where D (unitless) is the duty-cycle of MT, given by:

D =

VOUT

VIN

In rare cases where the minimum on-time restriction is

violated, the frequency of the LTM4653 automatically and

gradually folds back down to approximately one-fifth of its

programmed switching frequency to allow VOUT to remain

in regulation. See the Frequency Adjustment section. Be

remindedofNotes2,3and5intheElectricalCharacteristics

section regarding output current guidelines.

Input Capacitors

TheLTM4653achieveslowinputconductedEMInoisedue

to tight layout and high-frequency bypassing of MOSFETs

MT and MB within the module itself. A small filter inductor

(400nH) is integrated in the input line (from VIN to VD),

providing further noise attenuation—again, local to the

switching MOSFETs. The VD and VIN pins are available

for external input capacitors—CD and CINH—to form a

high-frequency π filter. As shown in the Simplified Block

Diagram, the ceramic capacitor CD on the LTM4653’s VD

pins handles the majority of the RMS current into the DC/

DC converter power stage and requires careful selection,

for that reason.

See Figures 4 to 6 for demonstration of LTM4653’s EMI

performance,meetingtheradiatedemissionsrequirements

of EN55022B.

The input capacitance, CD, is needed to filter the pulsed

current drawn by MT. To prevent excessive voltage sag

on VD, a low-effective series resistance (low-ESR, such

as an X7R ceramic) input capacitor should be used, sized

appropriately for the maximum CD RMS ripple current:

ICD(RMS) =

IOUT(MAX)

η%

• D • (1– D)

where η%istheestimatedefficiencyofthepowermodule.

(See Typical Performance Characteristics graphs.)

Several capacitors may be paralleled to meet the applica-

tion’s target size, height, and CDRMSripplecurrentrating.

For lower input voltage applications, sufficient bulk input

capacitance is needed to counteract line sag and transient

OPERATION

effects during output load changes. The bulk capacitor can

be a switcher-rated aluminum electrolytic capacitor or a

Polymer capacitor. Suggested values for CD and CINH are

found in Table 7.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LTM4653’s VIN,

SVIN,andVDpins.Aceramicinputcapacitorcombinedwith

trace or cable inductance forms a high Q (underdamped)

tank circuit. If the LTM4653 circuit is plugged into a live

supply, the input voltage can ring to twice its nominal

value, possibly exceeding the device’s rating. This situa-

tion is easily avoided; see the Hot-Plugging Safely section.

Output Capacitors

Output capacitors COUTH and COUTL are applied to VOUT of

theLTM4653.SufficientcapacitanceandlowESRarecalled

for, to meet the output voltage ripple, loop stability, and

transient requirements. COUTL can be a low ESR tantalum

or polymer capacitor. COUTH is a ceramic capacitor. The

typical output capacitance is 22μF (type X5R material, or

better), if ceramic-only output capacitors are used.

Table 7 shows a matrix of suggested output capacitors

optimized for 2A transient step-loads applied at 2A/μs.

Additional output filtering may be required by the system

designer, if further reduction of output ripple or dynamic

transient spike is required. The LTpowerCAD design tool is

availablefortransientandstabilityanalysis.Stabilitycriteria

are considered in the Table 7 matrix, and LTpowerCAD is

available 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

afunctionofstabilityandtransientresponse.LTpowerCAD

can be used to calculate the output ripple reduction as the

number of implemented phases increases by N times.

External loop compensation can be applied to COMPa if

needed, for transient response optimization.

Forced Continuous Operation

LeavetheCLKINpinopencircuittocommandtheLTM4653

for forced continuous operation. In this mode, the control

loop is allowed to command the inductor peak current to

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