LTC3869 datasheet(23/50 Pages) LINER

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LTC3839

3839fa

Figure 6. Circuit Model for Input Capacitor

Ripple Current Simulation

APPLICATIONS INFORMATION

higher temperature than required. Several capacitors may

also be paralleled to meet size or height requirements in

the design. Due to the high operating frequency of the

LTC3839, additional ceramic capacitors should also be

used in parallel for CIN close to the IC and power switches

to bypass the high frequency switching noises. Typically

multiple X5R or X7R ceramic capacitors are put in parallel

with either conductive-polymer or aluminum-electrolytic

types of bulk capacitors. Because of its low ESR, the

ceramic capacitors will take most of the RMS ripple cur-

rent. Vendors do not consistently specify the ripple current

rating for ceramics, but ceramics could also fail due to

excessive ripple current. Always consult the manufacturer

if there is any question.

Figure 6 represents a simplified circuit model for calculat-

ing the ripple currents in each of these capacitors. The

input inductance (LIN) between the input source and the

input of the converter will affect the ripple current through

the capacitors. A lower input inductance will result in less

ripple current through the input capacitors since more

ripple current will now be flowing out of the input source.

OS-CON) can be used for its higher ripple current rating

and lower ESR. For a wide VIN range that also require

higher voltage rating, aluminum-electrolytic capacitors are

more attractive since it can provide a larger capacitance

for more damping. An aluminum-electrolytic capacitor

with a ripple current rating that is high enough to handle

all of the ripple current by itself will be very large. But

when in parallel with ceramics, an aluminum-electrolytic

capacitor will take a much smaller portion of the RMS

ripple current due to its high ESR. However, it is crucial

that the ripple current through the aluminum-electrolytic

capacitor should not exceed its rating since this will

produce significant heat, which will cause the electrolyte

inside the capacitor to dry over time and its capacitance

to go down and ESR to go up.

The benefit of PolyPhase operation is reduced RMS cur-

rents and therefore less power loss on the input capaci-

tors. 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 PolyPhase

system. The details of a close form equation can be found

in Application Note 77 “High Efficiency, High Density,

PolyPhase Converters for High Current Applications”.

Figure 7 shows the input capacitor RMS ripple currents

normalized against the DC output currents with respect

to the duty cycle. This graph can be used to estimate the

maximum RMS capacitor current for a multiple-phase

application, assuming the channels are identical and their

phases are fully interleaved.

–

LIN

1μH

ESR(BULK)

VIN

ESL(BULK)

CIN(BULK)

ESR(CERAMIC)

ESL(CERAMIC)

IPULSE(PHASE1)

CIN(CERAMIC)

IPULSE(PHASE2)

3839 F06

Figure 7. Normalized RMS Input Ripple Current

DUTY FACTOR (VO/VIN)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.6

0.5

0.4

0.3

0.2

0.1

3839 F07

6-PHASE

4-PHASE

3-PHASE

2-PHASE

1-PHASE

For simulations with this model, look at the ripple current

during steady-state for the case where one phase is fully

loaded and the other was not loaded. This will in general

be the worst case for ripple current since the ripple cur-

rent from one phase will not be cancelled by ripple current

from the other phase.

Note that the bulk capacitor also has to be chosen for

RMS rating with ample margin beyond its RMS current

per simulation with the circuit model provided. For a lower

VIN range, a conductive-polymer type (such as Sanyo

LTC3869 Datasheet(PDF) 23 Page - Linear Technology

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