LTC4449 datasheet(42/108 Pages) LINER

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LTC3882-1

Rev A

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APPLICATIONS INFORMATION

Efficiency Considerations

Normally, one of the primary goals of any LTC3882-1 ap-

plication will be to obtain the highest practical conversion

efficiency. The efficiency of a switching regulator is equal

to the output power divided by the input power. It is often

useful to analyze individual losses to determine what is

limitingtheefficiencyandtoascertainwhichchangewould

producethemostimprovement.Balancingorlimitingthese

individual losses plays a dominant role in the component

selection process outlined over the next few sections.

Percent efficiency can be expressed as:

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

where L1, L2, et al, are the individual losses as a percent-

age of input power: 100 • PLn /PIN.

Although all dissipative elements in the system produce

losses, four main sources usually account for most of

the losses in LTC3882-1 applications: IC supply current,

I2R losses, topside power MOSFET transition losses and

total gate drive current.

1. The LTC3882-1 IC supply current is a DC value given

in the Electrical Characteristics table. The absolute loss

created by the IC itself is approximately this current

timestheVCCsupplyvoltage.ICsupplycurrenttypically

results in a small loss (<0.1%).

2. I2R losses occur mainly in the DC resistances of the

MOSFET, inductor, PCB routing, and input and output

capacitor ESR. Since each MOSFET is only on for part

of the cycle, its on-resistance is effectively multiplied by

the percentage of the cycle it is on. Therefore the bot-

tom MOSFET should have a much lower on-resistance

RDS(ON) than the top MOSFET in high step-down ratio

applications. It is crucial that careful attention is paid

to the layout of the power path on the PCB to minimize

its resistance. In a 2-phase 1.2V system, 1mΩ of PCB

resistance at the output costs 5% in efficiency with the

output running at 60A.

3. Transition losses apply only to the topside MOSFET and

becomesignificantwhenoperatingathighinputvoltages

(typically above 12V). This loss can be minimized by

choosing a driver with very low drive resistance and a

MOSFET with low gate charge QG, gate resistance RG

andMillercapacitanceCMILLER.Absolutetransitionloss

can be estimated by:

PTRANS = (1.7) • VIN2 • IOUT • CMILLER • fPWM

4. Gate drive current is equal to the sum of the top and bot-

tomMOSFETgatechargesmultipliedbythefrequencyof

operation. These charges are based on the gate voltage

applied by the FET driver and can be determined from

manufacturer curves like the one shown in Figure 21

Many driver ICs employ asymmetrical gate voltages for

top and bottom FETs.

Other sources of loss include body or Schottky diode

conduction during the driver dependent non-overlap time

and inductor core losses. These latter categories generally

account for less than 2% total additional loss.

PWM Frequency and Inductor Selection

TheselectionofthePWMswitchingfrequencyisatrade-off

betweenefficiency,transientresponseandcomponentsize.

High frequency operation reduces the size of the inductor

and output capacitor as well as increasing the maximum

practical control loop bandwidth. However, efficiency is

generally lower due to increased transition and switch-

ing losses. The inductor value is related to the switching

frequency fPWM and step-down ratio. It should be selected

to meet choke ripple current requirements. The inductor

value can be calculated using the following equation:

L =

VOUT

f PWM • ΔIL

⎛

⎝⎜

⎞

⎠⎟

• 1–

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

Allowing a larger value of choke ripple current (∆IL) leads

to smaller L, but results in greater core loss and higher

output voltage ripple for a given output capacitance and/

or ESR. A reasonable starting point for setting the ripple

current is 30% of the maximum output current.

The inductor saturation current rating needs to be higher

thanthepeakinductorcurrentduringtransientconditions.

If IOUT is the maximum rated load current, then the maxi-

mum transient current IMAX would normally be chosen to

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

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