LTC3892-1_15 Datasheet, PDF(26/36 Page) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

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LTC3892-1_15 Datasheet, PDF (26/36 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

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LTC3892/LTC3892-1

Applications Information

that the LTC3892/LTC3892-1 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 LTC3892/LTC3892-1 is

approximately 80ns. 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% 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 LTC3892/LTC3892-1 circuits: 1) IC VIN current,

2) DRVCC 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. DRVCC 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 DRVCC to ground. The resulting dQ/dt is a cur-

rent out of DRVCC 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 DRVCC 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 DRVCC 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 resis-

tance of one MOSFET can simply be summed with the

resistances of L, RSENSE and ESR to obtain I2R losses.

and 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!

4. Transition losses apply only to the top MOSFET(s), and

become significant only when operating at high input

For more information www.linear.com/LTC3892

38921f

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