LTC3621 datasheet(12/20 Pages) AD

12 / 20 page

LTC3644/LTC3644-2

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Frequency Sync Capability

The LTC3644 has the capability to sync to a ±50% range

of the internal programmed frequency. Once engaged in

sync, the LTC3644 immediately runs at the external clock

frequency in forced continuous mode.

Inductor Selection

Given the desired input and output voltages, the inductor

valueandoperatingfrequencydeterminetheripplecurrent:

ΔIL =

VOUT

f •L

−

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

Lower ripple current reduces power losses in the inductor,

ESR losses in the output capacitors and output voltage

ripple. Highest efficiency operation is obtained at low

frequency with small ripple current. However, achieving

this requires a large inductor. There is a trade-off between

component size, efficiency and operating frequency.

A reasonable starting point is to choose a ripple current

this is about 40% of IOUT(MAX). When calculating the

ripple current, IOUT(MAX) refers to the maximum output

current of the regulator, not the maximum load current of

the intended application. To guarantee that ripple current

does not exceed a specified maximum, the inductance

should be chosen according to:

VOUT

f •

ΔIL(MAX)

−

VOUT

VIN(MAX)

⎛

⎝

⎜

⎞

⎠

⎟

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

be selected. Actual core loss is independent of core size

for a fixed inductor value, but is very dependent on the

inductance selected. As the inductance or frequency

in-creases, core loss decrease. Unfortunately, increased

inductance requires more turns of wire and therefore

copper losses increase.

Ferritedesignshaveverylowcorelossesandarepreferred

at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard,” which means that

the inductance collapses abruptly when the peak design

current is exceeded. This results in an abrupt increase in

inductor ripple current and consequently output voltage

ripple. Do not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy

materials are small and don’t radiate much energy, but

generally cost more than powdered iron core inductors

with similar characteristics. The choice of which style

inductor to use mainly depends on the price versus size

requirements and any radiated field/EMI requirements.

New designs for surface mount inductors are available

from Coilcraft, Murata, Vishay, TDK and Würth Elektronik.

Refer to Table 2 to Table 4 for more details.

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Percent efficiency can

be expressed as:

% Efficiency = 100% - (L1 + L2 + L3 + …)

where L1, L2 etc. are the individual losses as a percentage

of input power. Although all dissipative elements in the

circuit produce losses, three main sources in the LTC3644

circuit are: 1) I2R losses, 2) switching and biasing losses,

3) other losses.

1. I2R losses are calculated from the DC resistances

of the internal switches, RSW, and external inductor,

RL. In continuous mode, the average output current

flows through inductor L but is “chopped” between

theinternaltopandbottompowerMOSFETs.Thus,the

series resistance looking into the SW pin is a function

of both the top and bottom MOSFET RDS(ON) and the

duty cycle (DC) as follows:

RSW = (RDS(ON)TOP)(DC)+(RDS(ON)BOT)(1 – DC)

TheRDS(ON)forboththetopandbottomMOSFETscanbe

obtainedfromtheTypicalPerformanceCharacteristics

curves. Thus to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

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