LTC3727LX-1_15 Datasheet, PDF(21/28 Page) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

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LTC3727LX-1_15 Datasheet, PDF (21/28 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

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LTC3727LX-1

APPLICATIO S I FOR ATIO

complete explanation is included in Design Solutions 10

(see www.linear.com).

INTVCC

RT2

RT1

ITH

LTC3727LX-1

3727LX1 F08

Figure 8. Active Voltage Positioning Applied to the LTC3727LX-1

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% â (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, four main sources usually account for most of the

losses in LTC3727LX-1 circuits: 1) LTC3727LX-1 VIN

current (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 cur-

rents; 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 current. 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 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, current sense resis-

tor, and input and output capacitor ESR. In continuous

mode the average output current flows through L and

RSENSE, but is âchoppedâ between the topside MOSFET

and the synchronous MOSFET. If the two MOSFETs

have approximately the same RDS(ON), then the resis-

tance 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) = 30mâ¦, RL = 50mâ¦,

RSENSE = 10mâ¦ and RESR = 40mâ¦ (sum of both input

and output capacitance losses), then the total resis-

tance is 130mâ¦. 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 âhiddenâ 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 âsystemâ 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 switch-

ing frequency. A 25W supply will typically require a mini-

3727lx1fa

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