LTC3415 datasheet(18/36 Pages) LINER

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LTC3613

3613fa

APPLICATIONS INFORMATION

be placed close to each ITH pin. If a compensation scheme

is stable on a single phase application, a polyphase ap-

plication with N phases should be compensated as:

CITH1 = N • CITH(SINGLE), CITH2 = N • CITH2(SINGLE) and

RITH = RITH(SINGLE)/N.

The TRACK/SS pins should be connected together so

that all LTC3613s start up with the same slew rate. The

VOSENSE+ pins of paralleled LTC3613s should be connected

together to prevent any false triggering of overvoltage and

short circuit protection. Only one divider is necessary. The

remote output and ground traces should be routed together

as differential pairs and terminated at the same remote

sensing location (preferably Kelvin connected across the

bulk capacitors at the remote output point). The smaller

value ceramic input and output capacitors, however, should

be in close proximity to the ICs.

CIN and COUT Selection

In continuous mode, the current into PVIN is a square wave

of duty cycle VOUT/VIN. To prevent large voltage transients,

a low ESR input capacitor sized for the maximum RMS

current must be used. The maximum RMS capacitor cur-

rent is given by:

IRMS ≅IOUT(MAX) •

VOUT

VIN

•

VIN

VOUT

–1

This formula has a maximum at VIN = 2VOUT, where IRMS

= IOUT(MAX)/2. This simple worst-case condition is com-

monly used for design because even significant deviations

do not offer much relief. Note that capacitor manufactur-

ers’ ripple current ratings for electrolytic and conductive

polymer capacitors are often based on only 2000 hours of

life. This makes it advisable to further derate the capacitor

or to choose a capacitor rated at a higher temperature

than required.

The selection of COUT is primarily determined by the effec-

tive series resistance, ESR, to minimize voltage ripple. The

output ripple, ΔVOUT,incontinuousmodeisdeterminedby:

ΔV

OUT ≤ΔIL RESR +

• f•C

OUT

⎛

⎝⎜

⎞

⎠⎟

The output ripple is highest at maximum input voltage

since ΔIL increases with input voltage. Typically, once the

ESR requirement for COUT has been met, the RMS current

rating generally far exceeds the peak-to-peak current ripple

requirement. The choice of using smaller output capaci-

tance increases the ripple voltage due to the discharging

term but can be compensated for by using capacitors of

very low ESR to maintain the ripple voltage.

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount pack-

ages. Special polymer capacitors offer very low ESR but

have lower capacitance density than other types. Tantalum

capacitors have the highest capacitance density but it is

important to only use types that have been surge tested

for use in switching power supplies. Aluminum electrolytic

capacitors have significantly higher ESR, but can be used

in cost-sensitive applications provided that consideration

is given to ripple current ratings and long-term reliability.

Ceramic capacitors have excellent low ESR characteristics

but can have a high voltage coefficient and audible piezo-

electric effects. The high Q of ceramic capacitors with trace

inductance can also lead to significant ringing. When using

ceramic input capacitors, care must be taken to ensure

that ringing from inrush currents and switching does not

pose an overvoltage hazard to the regulator.

For high switching frequencies, reducing output ripple and

better EMI filtering may require small-value capacitors that

have low ESL (and correspondingly higher self resonant

frequencies) to be placed in parallel with larger value

capacitors that have higher ESL. This will ensure good

noise and EMI filtering in the entire frequency spectrum

of interest. Even though ceramic capacitors generally

have good high frequency performance, small ceramic

capacitors may still have to be parallel connected with

large ones to optimize performance.

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