LTC3733 datasheet(27/32 Pages) LINER

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LTC3733/LTC3733-1

3733f

APPLICATIO S I FOR ATIO

The worst-case input RMS ripple current for a single stage

design peaks at twice the value of the output voltage. The

worst-case input RMS ripple current for a two stage

design results in peaks at 1/4 and 3/4 of the input voltage,

and the worst-case input RMS ripple current for a three

stage design results in peaks at 1/6, 1/2, and 5/6 of the

input voltage. The peaks, however, are at ever decreasing

levels with the addition of more phases. A higher effective

duty factor results because the duty factors “add” as long

as the currents in each stage are balanced. Refer to AN19

for a detailed description of how to calculate RMS current

for the single stage switching regulator.

Figure 6 illustrates the RMS input current drawn from the

input capacitance versus the duty cycle as determined by

the ration of input and output voltage. The peak input RMS

current level of the single phase system is reduced by 2/3

in a 3-phase solution due to the current splitting between

the three stages.

The output ripple current is reduced significantly when

compared to the single phase solution using the same

inductance value because the VOUT/L discharge currents

term from the stages that has their bottom MOSFETs on

subtract current from the (VCC – VOUT)/L charging current

resulting from the stage which has its top MOSFET on. The

output ripple current for a 3-phase design is:

IP-P = ()( )()

OUT

–

The ripple frequency is also increased by three, further

reducing the required output capacitance when VCC<3VOUT

as illustrated in Figure 4.

Efficiency Calculation

To estimate efficiency, the DC loss terms include the input

and output capacitor ESR, each MOSFET RDS(ON), induc-

tor resistance RL, the sense resistance RSENSE and the

forward drop of the Schottky rectifier at the operating

output current and temperature. Typical values for the

design example given previously in this data sheet are:

Main MOSFET RDS(ON) = 7mΩ (9mΩ at 90°C)

Sync MOSFET RDS(ON) = 7mΩ (9mΩ at 90°C)

CINESR = 20mΩ

COUTESR = 3mΩ

RL = 2.5mΩ

RSENSE = 3mΩ

VSCHOTTKY = 0.8V at 15A (0.7V at 90°C)

VOUT = 1.3V

VIN = 12V

IMAX = 0.8V at 15A (0.7V at 90°C)

δ = 0.01%°C

N = 3

f = 400kHz

The main MOSFET is on for the duty factor VOUT/VIN and

the synchronous MOSFET is on for the rest of the period

or simply (1 – VOUT/VIN). Assuming the ripple current is

small, the AC loss in the inductor can be made small if a

good quality inductor is chosen. The average current,

IOUT is used to simplify the calaculations. The equation

below is not exact but should provide a good technique

for the comparison of selected components and give a

result that is within 10% to 20% of the final application.

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