LTC3707IGN datasheet(23/32 Pages) LINER

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LTC3707

3707fb

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 LTC3707 circuits: 1) LTC3707 VIN cur-

rent (including loading on the 3.3V internal regulator),

2) INTVCC regulator current, 3) I2R losses, 4) Topside

MOSFET transition losses.

1. The VIN current has two components: the ﬁrst is the DC

supply current given in the Electrical Characteristics table,

which excludes MOSFET driver and control currents; the

second is the current drawn from the 3.3V linear regulator

output. 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 from INTVCC

to ground. The resulting dQ/dt is a current out of INTVCC

that is typically much larger than the control circuit cur-

rent. In continuous mode, IGATECHG =f(QT+QB), where QT

and QB are the gate charges of the topside and bottom

side MOSFETs.

Supplying INTVCC power through the EXTVCC switch input

from an output-derived source 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 ap-

plication, 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, current sense resis-

tor, and input and output capacitor ESR. In continuous

mode the average output current ﬂows through L and

RSENSE, but is “chopped” between the topside MOSFET

and the synchronous MOSFET. If the two MOSFETs have

approximately the same RDS(ON), then the resistance of

one MOSFET can simply be summed with the resistances

of L, RSENSE and ESR to obtain I2R losses. For example, if

each RDS(ON) = 30mΩ, RL = 50mΩ, RSENSE = 10mΩ and

RESR = 40mΩ (sum of both input and output capacitance

losses), then the total resistance is 130m

Ω. This results

in losses ranging from 3% to 13% as the output current

increases from 1A to 5A for a 5V output, or a 4% to 20%

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 and higher currents required by

high performance digital systems is not doubling but

quadrupling the importance 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 20m

Ω to 50mΩ of ESR. The LTC3707 2-

phase architecture typically halves this input capacitance

requirement over competing solutions. Other losses

including Schottky conduction losses during dead-time

and inductor core losses generally account for less than

2% total additional loss.

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