LTC1628 datasheet(16/32 Pages) LINER

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LTC1628/LTC1628-PG

Medium voltage (20V to 35V) ceramic, tantalum, OS-CON

and switcher-rated electrolytic capacitors can be used as

input capacitors, but each has drawbacks: ceramic voltage

coefficients are very high and may have audible piezoelec-

tric effects; tantalums need to be surge-rated; OS-CONs

suffer from higher inductance, larger case size and limited

surface-mount applicability; electrolytics’ higher ESR and

dryout possibility require several to be used. Multiphase

systems allow the lowest amount of capacitance overall.

As little as one 22

µF or two to three 10µF ceramic capaci-

tors are an ideal choice in a 20W to 35W power supply due

to their extremely low ESR. Even though the capacitance

at 20V is substantially below their rating at zero-bias, very

low ESR loss makes ceramics an ideal candidate for

highest efficiency battery operated systems. Also con-

sider parallel ceramic and high quality electrolytic capaci-

tors as an effective means of achieving ESR and bulk

capacitance goals.

In continuous mode, the source current of the top N-chan-

nel MOSFET 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 of one channel must

be used. The maximum RMS capacitor current is given by:

quiredI

RMS

MAX

OUT

≈

−

()

[]12

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

IRMS = IOUT/2. This simple worst case condition is com-

monly 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 LTC1628 multiphase can be calculated

by using the equation above for the higher power control-

ler and then calculating the loss that would have resulted

if both controller channels switch 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 control-

ler 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 multiphase 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 testing. 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 (

∆VOUT) is determined by:

∆∆

I ESR

OUT

≈+









Where f = operating frequency, COUT = output capacitance,

and

∆IL= ripple current in the inductor. The output ripple

is highest at maximum input voltage since

∆IL increases

with input voltage. With

∆IL = 0.3IOUT(MAX) the output

ripple will typically be less than 50mV at max VIN assum-

ing:

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 guaran-

tees that the output capacitance does not significantly

discharge during the operating frequency period due to

ripple current. 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 at or below

50mV. The ITH pin OPTI-LOOP compensation compo-

nents can be optimized to provide stable, high perfor-

mance transient response regardless of the output capaci-

tors selected.

APPLICATIO S I FOR ATIO

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