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LTC3728L-1

3728l1fc

APPLICATIONS INFORMATION

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

= IOUT/2. This simple worst case condition is commonly

used for design because even signiﬁcant deviations do not

offer much relief. Note that capacitor manufacturer’s ripple

current ratings 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. Several capacitors may also be paralleled to meet

size or height requirements in the design. Always consult

the manufacturer if there is any question.

The beneﬁt of the LTC3728L-1 multiphase clocking can

be calculated by using the equation above for the higher

power controller and then calculating the loss that would

have resulted if both controller channels switched on at

the same time. The total RMS power lost is lower when

both controllers are operating due to the interleaving of

current pulses through the input capacitor’s ESR. This is

why the input capacitor’s requirement calculated above for

the worst-case controller is adequate for the dual controller

design. Remember that input protection fuse resistance,

battery resistance and PC board trace resistance losses are

also reduced due to the reduced peak currents in a multi-

phase system.

The overall beneﬁt of a multiphase design

will only be fully realized when the source impedance of

the power supply/battery is included in the efﬁciency test-

ing. The drains of the two top MOSFETS should be placed

within 1cm of each other and share a common CIN(s).

Separating the drains and CIN may produce undesirable

voltage and current resonances at VIN.

The selection of COUT is driven by the required effective

series resistance (ESR). Typically once the ESR require-

ment is satisﬁed the capacitance is adequate for ﬁltering.

The output ripple (ΔVOUT) is determined by:

VOUT

IL ESR +

8fCOUT

Where f = operating frequency, COUT = output capacitance,

and ΔIL= ripple current in the inductor. The output ripple

is highest at maximum input voltage since ΔIL increases

with input voltage. With ΔIL = 0.3IOUT(MAX) the output

ripple will typically be less than 50mV at the maximum

VIN assuming:

COUT Recommended ESR < 2 RSENSE

and COUT > 1/(8fRSENSE)

The ﬁrst condition relates to the ripple current into the ESR

of the output capacitance while the second term guarantees

that the output capacitance does not signiﬁcantly discharge

during the operating frequency period due to ripple current.

The choice of using smaller output capacitance 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 at or below 50mV. The ITH pin

OPTI-LOOP compensation components can be optimized

to provide stable, high performance transient response

regardless of the output capacitors selected.

Manufacturers such as Nichicon, Nippon Chemi-Con and

Sanyo can be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest (ESR)(size)

product of any aluminum electrolytic at a somewhat

higher price. An additional ceramic capacitor in parallel

with OS-CON capacitors is recommended to reduce the

inductance effects.

In surface mount applications multiple capacitors may

need to be used in parallel to meet ESR, RMS cur-

rent handling and load step requirements. Aluminum

electrolytic, dry tantalum and special polymer capaci-

tors are available in surface mount packages. Special

polymer surface mount capacitors offer very low ESR

but have lower storage capacity per unit volume than

other capacitor types. These capacitors offer a very

cost-effective output capacitor solution and are an ideal

choice when combined with a controller having high

loop bandwidth. Tantalum capacitors offer the highest

capacitance density and are often used as output capaci-

tors for switching regulators having controlled soft-start.

Several excellent surge-tested choices are the AVX TPS,

AVX TPSV or the KEMET T510 series of surface mount

tantalums, available in case heights ranging from 2mm

to 4mm. Aluminum electrolytic capacitors can be used

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