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LTC3807

3807fa

For more information www.linear.com/LTC3807

charge required to turn on the top MOSFET. Low duty

cycle applications may approach this minimum on-time

limit and care should be taken to ensure that:

tON(MIN) <

VOUT

VIN f()

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple voltage and current will increase.

The minimum on-time for the LTC3807 is approximately

95ns. However, as the peak sense voltage decreases

the minimum on-time gradually increases up to about

130ns. This is of particular concern in forced continuous

applications with low ripple current at light loads. If the

duty cycle drops below the minimum on-time limit in this

situation, a significant amount of cycle skipping can occur

with correspondingly larger current and voltage ripple.

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 LTC3807 circuits: 1) IC VIN current, 2) IN-

TVCC 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

APPLICATIONS INFORMATION

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.

SupplyingINTVCCfromanoutput-derivedsourcepower

through EXTVCC 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 applica-

tion, 10mA of INTVCC current results in approximately

2.5mA of VIN current. This reduces the midcurrent 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

andthesynchronousMOSFET.IfthetwoMOSFETshave

approximately the same RDS(ON), then the resistance

of one MOSFET can simply be summed with the resis-

tances 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 130mΩ. 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

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

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