LTM4660 datasheet(17/28 Pages) AD

17 / 28 page

LTM4660

Rev. 0

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Without considering the inductor current ripple, the

RMS current of the input capacitor can be estimated by

Equation 5.

ICIN(RMS) =

IOUT(MAX)

2 •

η%

•

D • D´

(

)

(5)

where D is 2 • VOUT/VIN and 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 or Polymer capacitor.

Output Capacitor Selection

The LTM4660 is designed to achieve good transient

response and low output voltage ripple. Choose COUT

with low ESR to meet the output ripple and transient per-

formance requirements. Place at least two 10μF output

ceramic capacitors close to VOUT and GND pins. This low-

ers the output ripple by lowering the total ESR of COUT.

Aluminum electrolytic capacitors of 100μF or 150μF can

be placed further away as bulk output capacitors. Choose

enough bulk capacitance to meet the transient specification

demanded by downstream loads. When choosing COUT, be

aware of the impact of its ESR on loop stability. Refer to

LTM4660 model in LTspice to optimize and fine tune the

combination of COUT for each LTM4660 application.

CFLY and CMID Selection

For the LTM4660 μModule, capacitor banks CFLY and CMID

connected external to the μModule, are part of the energy

transfer elements. Therefore, ceramic capacitors are attrac-

tive since they have the lowest ESR. However, care should be

taken when choosing this type of capacitors. During operation

the DC voltage across CFLY and CMID is approximately half

the VIN supply, therefore the voltage rating of the capacitors

should be greater than that. As a general rule, select the volt-

age rating of the capacitor to be twice the operating voltage

of the capacitor. For the same voltage rating and capacitance,

a larger case size will have a lower failure rate.

In addition, the operating temperature of the capaci-

tors needs to be considered. For operating temperature

above 85°C, capacitors with the X7R dielectric need to

be used while X5R dielectric is adequate for operation

below 85°C. For long term reliability of the capacitor, keep

the temperature rise of the capacitor to be below 20°C,

preferably 10°C. The temperature rise of the capacitor is

dependent on the amount of RMS current through the

capacitor and the operating frequency.

Ceramic capacitors also have a large voltage coefficient, los-

ing close to half their capacitance when the DC bias across a

given capacitor is half its rated voltage. The DC bias effect on

a capacitor is greater when the case size is smaller. Factor in

these effects when deciding on the capacitance.

The ripple voltage on CFLY and CMID is given by Equation 6.

VRIPPLE =

IOUT

CBANK • fSW

2VOUT

VIN

(6)

where CBANK is either CFLY or CMID, IOUT is output current

and fSW is the switching frequency.

Ripple voltage on CFLY and CMID contributes significantly

to the power dissipated in LTM4660 (see Efficiency fig-

ures in Typical Performance Characteristics section). As a

good starting point, select enough capacitance such that

the ripple on each capacitor is less than 1% of the DC

bias voltage of the capacitor. For example, if the DC bias

voltage of the capacitor is 24V, keep the ripple to be less

than 240mV. To achieve lowest loss in LTM4660, select

the capacitance of CMID to be same as that of CFLY.

LTM4660 is designed to deliver 300W of output power

at 12VOUT. For maximum efficiency at 300W, use eight

to twelve 10μF ceramic capacitors each for CMID and

CFLY. Considering the voltage derating of ceramic capaci-

tors, for bus voltages in range of 48VIN to 54VIN, choose

ceramic capacitors rated to at least 50V.

Timer and Hysteresis

The LTM4660 μModule uses a switched capacitor hybrid

topology where the switched capacitor stage consists of CFLY

and CMID capacitorbanksconnectedalternatelyinseriesorin

parallel through the power switches inside LTM4660. During

start-up, CFLY andCMID capacitorsarecompletelydischarged.

Starting switching with the capacitors discharged may lead

to undesirably high currents through the power switches.

To circumvent this phenomenon, LTM4660 uses a patented

technology to balance the charge across the CFLY and CMID

to half of VIN during start-up.

APPLICATIONS INFORMATION

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