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LTC3890-2

38902f

APPLICATIONS INFORMATION

The minimum on-time for the LTC3890-2 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 applica-

tions 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 cor-

respondingly 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 LTC3890-2 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

Electrical Characteristics table, which excludes MOSFET

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

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 from an output-derived source power

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 application,

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

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 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

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) • VIN • 2 • IO(MAX) • CRSS • f

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