LTC3707IGN datasheet(16/32 Pages) LINER

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LTC3707

3707fb

APPLICATIONS INFORMATION

30% to 70% when compared to a single phase power

supply solution.

The type of input capacitor, value and ESR rating have

efﬁciency effects that need to be considered in the selec-

tion process. The capacitance value chosen should be

sufﬁcient to store adequate charge to keep high peak

battery currents down. 20μF to 40μF is usually sufﬁcient

for a 25W output supply operating at 200kHz. The ESR of

the capacitor is important for capacitor power dissipation

as well as overall battery efﬁciency. All of the power (RMS

ripple current • ESR) not only heats up the capacitor but

wastes power from the battery.

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

piezoelectric 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 22μF or two to three

10μF ceramic capacitors are an ideal choice in a 20W to

50W 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 consider parallel ceramic and high quality

electrolytic capacitors as an effective means of achieving

ESR and bulk capacitance goals.

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

MOSFET is a square wave of duty cycle VOUT/VIN.Toprevent

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:

IN Required IRMS ≈ IMAX

OUT VIN − VOUT

()

⎡⎣

⎤⎦

1/2

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 capaci-

tor, 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 LTC3707 multiphase 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 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 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 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

(ΔVOUT) is determined by:

ΔV

OUT ≈ΔIL ESR +

8fC

OUT

⎛

⎝⎜

⎞

⎠⎟

Where f = operating frequency, COUT = output capacitance,

and

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

highest at maximum input voltage since

ΔILincreaseswith

input voltage. With

ΔIL=0.3IOUT(MAX)theoutputripplewill

typically be less than 50mV at max 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

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