LTC3857-1_15 Datasheet, PDF(24/38 Page) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LTC3857-1_15 Datasheet, PDF (24/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

◁

LTC3857-1

APPLICATIONS INFORMATION

Minimum On-Time Considerations

Minimum on-time, tON(MIN), is the smallest time dura-

tion that the LTC3857-1 is capable of turning on the top

MOSFET. It is determined by internal timing delays and the

gate 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 LTC3857-1 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% â (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 LTC3857-1 circuits: 1) IC VIN current, 2) INTVCC

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

transition losses.

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

in the Electrical Characteristics table, which excludes

MOSFET driver and control currents. VIN current typi-

cally 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 power source

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

output capacitance losses), then the total resistance

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!

38571fc

▷