LTC1874 datasheet(8/12 Pages) LINER

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LTC1874

IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE

occurs at the maximum input voltage.

In Burst Mode operation on an LTC1874 controller, the

ripple current is normally set such that the inductor

current is continuous during the burst periods. Therefore,

the peak-to-peak ripple current must not exceed:

RIPPLE

SENSE

≤

003

This implies a minimum inductance of:

MIN

OUT

SENSE

OUT

−

















003

(Use VIN(MAX) = VIN)

A smaller value than LMIN could be used in the circuit;

however, the inductor current will not be continuous

during burst periods.

Inductor Core Selection

Once the value of inductor is known, an off the shelf

inductor can be selected. The inductor should be rated for

the calculated peak current. Some manufacturers specify

both peak saturation current and peak RMS current. Make

sure that the RMS current meets your continuous load

requirements. Also, you may want to compare the DC

resistance of different inductors in order to optimize the

efficiency.

Inductor core losses are usually not specified and you will

need to evaluate them yourself. Usually, the core losses

are not a problem because the inductors operate with

relatively low magnetic flux swings. The best way to

evaluate the core losses is by measuring the converters

efficiency. Converter efficiency will reveal the difference in

both DC current losses and core losses.

Off the shelf inductors are available from numerous manu-

facturers. Some of the most common manufacturers are

Coilcraft, Coiltronics, Panasonic, Toko, Tokin, Murata and

Sumida.

Power MOSFET Selection

The main selection criteria for the power MOSFET are the

threshold voltage VGS(TH), the “on” resistance RDS(ON),

reverse transfer capacitance CRSS and total gate charge.

Since the controller is designed for operation down to low

input voltages, a logic level threshold MOSFET (RDS(ON)

guaranteed at VGS = 2.5V) is required for applications that

work close to this voltage. When these MOSFETs are used,

make sure that the input supply to the controller is less

than the absolute maximum VGS rating, typically 8V.

The required minimum RDS(ON) of the MOSFET is gov-

erned by its allowable power dissipation. For applications

that may operate the controller in dropout, i.e., 100% duty

cycle, at its worst case the required RDS(ON) is given by:

DS ON

OUT MAX

()

= () +()

100

where PP is the allowable power dissipation and δp is the

temperature dependency of RDS(ON). (1 + δp) is generally

given for a MOSFET in the form of a normalized RDS(ON) vs

temperature curve, but

δp = 0.005/°C can be used as an

approximation for low voltage MOSFETs.

In applications where the maximum duty cycle is less than

100% and the controller is in continuous mode, the

RDS(ON) is governed by:

DC I

DS ON

OUT

() ≅ ()

()

where DC is the maximum operating duty cycle of the

controller.

Output Diode Selection

The catch diode carries load current during the off-time.

The average diode current is therefore dependent on the

MOSFET duty cycle. At high input voltages the diode

conducts most of the time. As VIN approaches VOUT the

diode conducts only a small fraction of the time. The most

stressful condition for the diode is when the output is

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