LTC3855 datasheet(21/44 Pages) LINER

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LTC3855

3855f

applicaTions inForMaTion

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX) for a duty cycle less than

40%. Note that the largest ripple current occurs at the

highestinputvoltage.Toguaranteethatripplecurrentdoes

not exceed a specified maximum, the inductor should be

chosen according to:

≥

VIN – VOUT

fOSC •IRIPPLE

•

VOUT

VIN

For duty cycles greater than 40%, the 10mV current

sense ripple voltage requirement is relaxed because the

slope compensation signal aids the signal-to-noise ratio

and because a lower limit is placed on the inductor value

to avoid subharmonic oscillations. To ensure stability for

duty cycles up to the maximum of 95%, use the following

equation to find the minimum inductance.

MIN

OUT

SW LOAD MAX

•

• .

(

)

1 4

where

LMIN is in units of µH

fSW is in units of MHz

Inductor Core Selection

Once the inductance value is determined, the type of in-

ductor must be selected. Core loss is independent of core

size for a fixed inductor value, but it is very dependent

on inductance selected. As inductance increases, core

losses go down. Unfortunately, increased inductance

requires more turns of wire and therefore copper losses

will increase.

Ferrite designs have very low core loss and are preferred

at high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

core material saturates “hard,” which means that induc-

tance collapses abruptly when the peak design current is

exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Power MOSFET and Schottky Diode

(Optional) Selection

Two external power MOSFETs must be selected for each

controller in the LTC3855: one N-channel MOSFET for the

top (main) switch, and one N-channel MOSFET for the

bottom (synchronous) switch.

The peak-to-peak drive levels are set by the INTVCC

voltage. This voltage is typically 5V during start-up

(see EXTVCC Pin Connection). Consequently, logic-level

threshold MOSFETs must be used in most applications.

The only exception is if low input voltage is expected (VIN

< 5V); then, sub-logic level threshold MOSFETs (VGS(TH)

< 3V) should be used. Pay close attention to the BVDSS

specification for the MOSFETs as well; most of the logic

level MOSFETs are limited to 30V or less.

Selection criteria for the power MOSFETs include the

on-resistance RDS(ON), Miller capacitance CMILLER, input

voltage and maximum output current. Miller capacitance,

CMILLER, can be approximated from the gate charge curve

usually provided on the MOSFET manufacturers’ data

sheet. CMILLER is equal to the increase in gate charge

along the horizontal axis while the curve is approximately

flat divided by the specified change in VDS. This result is

then multiplied by the ratio of the application applied VDS

to the gate charge curve specified VDS. When the IC is

operating in continuous mode the duty cycles for the top

and bottom MOSFETs are given by:

Main Switch Duty Cycle

VOUT

VIN

Synchronous Switch Duty Cycle

VIN – VOUT

VIN

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