LTC3829 datasheet(33/40 Pages) LINER

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LTC3829

3829f

applicaTions inForMaTion

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 percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC3829 circuits: 1) IC VIN current, 2) INTVCC

regulator current, 3) I2R losses, 4) topside MOSFET

transition losses.

1. The VIN current is the DC supply current given in the

ElectricalCharacteristicstable,whichexcludesMOSFET

driver and control currents. VIN current typically results

in a small (<0.1%) loss.

2. INTVCC current is the sum of the MOSFET driver and

control currents. The MOSFET driver current results

from switching the gate capacitance of the power

MOSFETs. Each time a MOSFET gate is switched from

low to high to low again, a packet of charge dQ moves

fromINTVCCtoground.TheresultingdQ/dtisacurrent

out of INTVCC that is typically much larger than the

controlcircuitcurrent.Incontinuousmode,IGATECHG=

f(QT + QB), where QT and QB are the gate charges of the

topside and bottom side MOSFETs. Supplying INTVCC

power through EXTVCCfromanoutput-derivedsource

will scale the VIN current required for the driver and

control circuits by a factor of (duty cycle)/(efficiency).

For example, in a 20V to 5V application, 10mA of

INTVCC current results in approximately 2.5mA of VIN

current. This reduces the mid-current loss from 10%

or more (if the driver was powered directly from VIN)

to only a few percent.

3. I2R losses are predicted from the DC resistances of

the fuse (if used), MOSFET, inductor and current sense

resistor.Incontinuousmode,theaverageoutputcurrent

flows through L and RSENSE, but is chopped between

the topside MOSFET and the synchronous MOSFET. If

thetwoMOSFETshaveapproximatelythesameRDS(ON),

then the resistance of one MOSFET can simply be

summed with the resistances of L and RSENSE to ob-

tain I2R losses. For example, if each RDS(ON) = 10mΩ,

RL = 10mΩ, RSENSE = 5mΩ, then the total resistance is

25mΩ. This results in losses ranging from 2% to 8%

as the output current increases from 3A to 15A for a 5V

output, or a 3% to 12% loss for a 3.3V output.

Efficiency varies as the inverse square of VOUT for the

same external components and output power level. The

combined effects of increasingly lower output voltages

andhighercurrentsrequiredbyhighperformancedigital

systemsisnotdoublingbutquadruplingtheimportance

of loss terms in the switching regulator system!

4. Transition losses apply only to the topside MOSFET(s),

and become significant only when operating at high

input voltages (typically 15V or greater). Transition

losses can be estimated from:

Transition Loss = (1.7) VIN2 • IO(MAX) • CRSS • f

Other hidden losses such as copper trace and internal

battery resistances can account for an additional 5%

to 10% efficiency degradation in portable systems. It

is very important to include these system level losses

during the design phase. The internal battery and fuse

resistance losses can be minimized by making sure that

CIN has adequate charge storage and very low ESR at

the switching frequency. A 25W supply will typically

require a minimum of 20µF to 40µF of capacitance

having a maximum of 20mΩ to 50mΩ of ESR. Other

losses including Schottky conduction losses during

dead time and inductor core losses generally account

for less than 2% total additional loss.

LTC3829 Datasheet(PDF) 33 Page - Linear Technology

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