LTM4677 datasheet(48/110 Pages) AD

48 / 110 page

LTC3888-1

Rev. 0

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The inductor saturation current rating needs to be higher

than the peak inductor current during transient condi-

tions. If IOUT is the maximum rated load current, then the

maximum transient current IMAX would normally be cho-

sen to be some factor greater than IOUT (e.g., 1.6 • IOUT).

The minimum saturation current rating should be chosen

to allow margin due to manufacturing and temperature

variation in the power stage output current monitor. A

reasonable ISAT value would be 2.2 • IOUT.

The programmed current limit IOUT_OC_FAULT_LIMIT

must be low enough to ensure that the inductor never

saturates and high enough to allow increased current dur-

ing transient conditions. For example, if:

ISAT = 2.2 • IOUT, and

IMAX = 1.6 • IOUT

a reasonable output current limit would be

IOUT_OC_FAULT_LIMIT = 1.8 • IOUT

Once the value of L is known, the type of inductor must

be selected. High efficiency converters generally can-

not afford the core losses found in low-cost powdered

iron cores, forcing the use of more expensive ferrite

or molypermalloy cores. Also, core losses decrease as

inductance increases. Unfortunately, increased induc-

tance requires more turns of wire, larger inductance and

larger copper losses.

Ferrite designs have very low core loss and are preferred

at high switching frequencies. However, these core mate-

rials exhibit hard saturation, causing an abrupt reduc-

tion in the inductance when the peak current capability is

exceeded. Do not allow the core to saturate.

CIN SELECTION

The input bypass capacitance for an LTC3888-1 power

circuit needs to have ESR low enough to keep the sup-

ply drop low as the top MOSFETs turn on, RMS current

capability adequate to withstand the ripple current at the

input, and a capacitance value large enough to maintain

the input voltage until the input supply can make up the

difference. Generally, a capacitor that meets the first two

requirements (particularly a non-ceramic type) will have

far more capacitance than is required to keep capacitance-

based droop under control.

The input capacitance voltage rating should be at least 1.4

times the maximum input voltage. Power loss due to ESR

occurs as I2R dissipation in the capacitor itself. The input

capacitor RMS current and its impact on any preceding

input network is reduced by PolyPhase architecture. It can

be shown that the worst case RMS current occurs when

only one phase is operating. The phase with the highest

(VOUT)(IOUT) product should be used to determine the

maximum RMS current requirement. Increasing the num-

ber of phases will decrease the input RMS ripple current

from this maximum value. 2-phase operation typically

reduces the input capacitor RMS ripple current by a factor

of 30% to 70%.

In continuous inductor conduction mode, the source cur-

rent of the top power MOSFET is approximately a square

wave of duty cycle VOUT/VIN. The maximum RMS capaci-

tor current in this case is given by:

IRMS � IOUT(MAX)

VOUT VIN – VOUT

(

)

VIN

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

IRMS = IOUT/2

This simple worst-case condition is commonly used for

design because even significant deviations do not offer

much relief.

Note that manufacturer ripple current ratings for capaci-

tors 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.

Ceramic, tantalum, semiconductor electrolyte (OS-CON),

hybrid conductive polymer (SUNCON) and switcher-rated

electrolytic capacitors can be used as input capacitors, but

each has drawbacks. Ceramics have high voltage coeffi-

cients of capacitance and may have audible piezoelectric

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

fer from higher inductance, larger case size and limited

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