LTM4613 datasheet(14/34 Pages) LINER

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LTM4651

4651f

For more information www.linear.com/LTM4651

APPLICATIONS INFORMATION

Input Capacitors

TheLTM4651achieveslowinputconductedEMInoisedue

to tight layout and high-frequency bypassing of MOSFETs

MT and MB within the module itself. A small filter induc-

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

provides further noise attenuation—again, local to the

switching MOSFETs. The VD and VIN pins are available

for external input capacitors—VD and VINH—to form a

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

Diagram, the ceramic capacitor CD on the LTM4651’s VD

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

DC converter power stage and requires careful selection,

for that reason.

TomeettheradiatedemissionsrequirementsofEN55022B,

anadditionalfiltercapacitor,CINOUT,isneeded—connecting

fromVINtoVOUT–.SeeFigures5to 8forEMIperformance.

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) input

capacitor should be used, sized appropriately for the

maximum CD RMS ripple current:

ICD(RMS) =IPK • D• (1–D)

(6)

ICD(RMS) is maximum for D = 1/2. For D = 1/2,

ICD(RMS) = 1/2 • IPK or 3A. This simplification of the worst-

case condition is commonly used for design purposes

because even significant deviations in D do not offer much

relief, in practice. Furthermore: note that ripple current

ratings from capacitor manufacturers are often based on

2000 hours of life; therefore, it is advisable to significantly

over-designCD,and/orchooseacapacitorratedatahigher

temperature than required. Err on the side of caution and

contact the capacitor manufacturer to understand the

capacitor vendor’s derating methodology.

Several capacitors may be paralleled to meet the applica-

tion’s targetsize,height,andCDRMSripplecurrentrating.

For lower input voltage applications, sufficient bulk input

capacitance is needed for CINL to counteract line sag and

transient effects during output load changes. Suggested

values for CD and CINH are found in Table 8. Take note

that CD is connected from VD to VOUT–, whereas CINH and

CINL are connected from VIN to PGND; this is deliberate.

A final precaution regarding ceramic capacitors concerns

the maximum input voltage rating of the LTM4651’s VIN,

SVIN,andVDpins.Aceramicinputcapacitorcombinedwith

trace or cable inductance forms a high Q (underdamped)

tank circuit. If the LTM4651 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 COUTHandCOUTLareappliedtoVOUT–of

theLTM4651: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.

For highest reliability designs, polarized output capacitors

(VOUTL) are not recommended, as there is a possibility of

a diode-drop of reverse voltage appearing transiently on

VOUT– during rapid application of input voltage or when

RUNistoggledlogichigh(seeFigures33).Whenpolarized

capacitorsareusedonVOUT–,contactthecapacitorvendor

to understand what reverse voltage their polarized capaci-

tor can withstand. Be advised, polarized capacitor reverse

voltage rating is sometimes temperature-dependent.

Output voltage ripple (∆VOUT(PK-PK)–) is governed by

charge lost in COUTH and COUTL while MT is on, in addition

to the contribution of a resistive drop across the ESR of

the output capacitors. This is expressed by:

∆VOUT(PK–PK) ≈

ILOAD •D

COUT • fSW

+ILOAD •ESR

(7)

Table 8 shows a matrix of suggested output capacitors

optimized for transient step-loads that are 50% of the full

load capability for that combination of VIN,VOUT–,andfSW.

The table optimizes total equivalent ESR and total bulk

capacitancetoyieldthestatedtransient-loadperformance.

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 available for transient and stability analysis.

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