LTC3880 datasheet(24/50 Pages) LINER

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LTC3839

3839fa

APPLICATIONS INFORMATION

Figure 7 shows that the use of more phases will reduce

the ripple current through the input capacitors due to

ripple current cancellation. However, since LTC3839 is

only truly phase-interleaved at steady state, transient RMS

currents could be higher than the curves for the designated

number of phase. Therefore, it is advisable to choose

capacitors by taking account the specific load situations

of the applications. It is always the safest to choose input

capacitors’ RMS current rating closer to the worst case of

a single-phase application discussed above, calculated by

assuming the loss that would have resulted if controller

channels switched on at the same time.

However, it is generally not needed to size the input capaci-

tor for such worst-case conditions where on-times of the

phases coincide all the time. During a load step event, the

overlap of on-time will only occur for a small percentage

of time, especially when duty cycles are low. A transient

event where the switch nodes align for several cycles at

a time should not damage the capacitor. In most applica-

tions, sizing the input capacitors for 100% steady-state

load should be adequate. For example, a microprocessor

load may cause frequent overlap of the on-times, which

makes the ripple current higher, but the load current may

rarely be at 100% of IOUT(MAX). Using the worst-case load

current should already have margin built in for transient

conditions.

The VIN sources of the top MOSFETs should be placed

close to each other and share common CIN(s). Separating

the sources and CIN may produce undesirable voltage and

current resonances at VIN.

A small (0.1μF to 1μF) bypass capacitor between the IC’s

VIN pin and ground, placed close to the IC, is suggested.

A 2.2Ω to 10Ω resistor placed between CIN and the VIN

pin is also recommended as it provides further isolation

from switching noise of the two channels.

COUT Selection

The selection of output capacitance COUT is primarily

determined by the effective series resistance, ESR, to

minimize voltage ripple. The output voltage ripple

∆VOUT,

in continuous mode is determined by:

ΔV

OUT ≤ΔIL RESR +

8• f • C

OUT

⎛

⎝⎜

⎞

⎠⎟

where f is operating frequency, and

∆IL is ripple content in

the sum of inductor currents of all phases. The output ripple

is highest at maximum input voltage since

∆IL increases

with input voltage. Typically, once the ESR requirement

for COUT has been met, the RMS current rating generally

far exceeds that required from ripple current.

For LTC3839’s multiphase operation, it is advisable to

consider ripple requirements at specific load conditions

when determining the

∆IL for the output capacitor selec-

tion. At steady state, the LTC3839’s individual phases are

interleaved, and their ripples cancel each other at the output,

so ripple on COUT is reduced. During transient, when the

phases are not fully interleaved, the ripple cancellation

may not be as effective. For example, a large step of load

current increase may cause the phases to align for several

cycles, quickly ramping up the total inductor current and

pulling the VOUT from the droop. While the worst-case ∆IL

is the sum of the

∆IL’s of individual phases aligned during

a fast transient, such larger ripple lasts for only a short

time before the phase interleaving is restored.

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.

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount pack-

ages. Special polymer capacitors offer very low ESR but

have lower capacitance density than other types. Tantalum

capacitors have the highest capacitance density but it is

important to only use types that have been surge tested

for use in switching power supplies. Aluminum electrolytic

capacitors have significantly higher ESR, but can be used

in cost-sensitive applications provided that consideration

is given to ripple current ratings and long-term reliability.

Ceramic capacitors have excellent low ESR characteristics

but can have a high voltage coefficient and audible piezo-

electric effects. The high Q of ceramic capacitors with trace

inductance can also lead to significant ringing. When used

as input capacitors, care must be taken to ensure that ring-

ing from inrush currents and switching does not pose an

overvoltage hazard to the power switches and controller.

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