LTC3853_15 datasheet(22/36 Pages) LINER

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LTC3853

3853fc

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dual or triple controller design. Also, the input protection

fuse resistance, battery resistance, and PC board trace

resistance losses are also reduced due to the reduced

peak currents in a 3-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 testing. The sources of the top MOSFETs

should be placed within 1cm of each other and share a

common CIN(s). Separating the sources and CIN may pro-

duce undesirable voltage and current resonances at VIN.

A small (0.1µF to 1µF) bypass capacitor between the chip

VIN pin and ground, placed close to the LTC3853, is also

suggested.A2.2Ωto10ΩresistorplacedbetweenCINand

theVINpinprovidesfurtherisolationbetweenthechannels.

The selection of COUT is driven by the effective series

resistance (ESR). Typically, once the ESR requirement

is satisfied, the capacitance is adequate for filtering. The

output ripple (

DVOUT) is approximated by:

∆VOUT ≈IRIPPLE ESR +

8fCOUT









where f is the operating frequency, COUT is the output

capacitance and IRIPPLE is the ripple current in the induc-

tor. The output ripple is highest at maximum input voltage

since IRIPPLE increases with input voltage.

Setting Output Voltage

The LTC3853 output voltages are each set by an external

feedback resistive divider carefully placed across the out-

put, as shown in Figure 9. The regulated output voltage

is determined by:

VOUT = 0.8V • 1+









To improve the frequency response, a feed-forward ca-

pacitor, CFF, may be used. Great care should be taken to

route the VFB line away from noise sources, such as the

inductor or the SW line.

Figure 9. Setting Output Voltage

APPLICATIONS INFORMATION

1/3 LTC3853

VFB

VOUT

CFF

3853 F09

Fault Conditions: Current Limit and Current Foldback

The LTC3853 includes current foldback to help limit load

current when the output is shorted to ground. If the out-

put falls below 50% of its nominal output level, then the

maximum sense voltage is progressively lowered from its

maximumprogrammedvaluetoone-thirdofthemaximum

value. Foldback current limiting is disabled during the

soft-start or tracking up. Under short-circuit conditions

with very low duty cycles, the LTC3853 will begin cycle

skipping in order to limit the short-circuit current. In this

situation the bottom MOSFET will be dissipating most of

the power but less than in normal operation. The short-

circuit ripple current is determined by the minimum on-

time tON(MIN) of the LTC3853 (≈ 90ns), the input voltage

and inductor value:

∆IL(SC) = tON(MIN) •

VIN

The resulting short-circuit current is:

ISC =

1/3 VSENSE(MAX)

RSENSE

–

∆IL(SC)

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