SC415_08 datasheet(15/30 Pages) SEMTECH

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SC415

Figure 8

Design Procedure

Prior to designing a switch mode supply, the input

voltage, load current, and switching frequency must

be specified.

For notebook systems the maximum

input voltage (VINMAX) is determined by the highest

AC adaptor voltage, and the minimum input voltage

(VINMIN) is determined by the lowest battery voltage

after accounting for voltage drops due to connectors,

fuses and battery selector switches.

In general, four parameters are needed to define the

design:

Nominal output voltage (VOUT)

Static or DC output tolerance

Transient response

Maximum load current (IOUT)

Therearetwovaluesofloadcurrenttoconsider:continuous

load current and peak load current. Continuous load

current is concerned with thermal stresses which drive

the selection of input capacitors, MOSFETs and diodes.

Peak load current determines instantaneous component

stresses and filtering requirements such as inductor

saturation, output capacitors and design of the current

limit circuit.

Design example:

VIN = 10V min, 20V max

VOUT1 = 1.8V +/- 4%

Load = 10A maximum

Side1 will be used as an example.

Inductor Selection

Low inductor values result in smaller size but create

higher ripple current. Higher inductor values will reduce

the ripple current but are larger and more costly. Because

wire resistance varies widely for different inductors and

because magnetic core losses vary widely with operating

conditions, it is often difficult to choose which inductor

will optimize efficiency. The general rule is that higher

inductor values have better efficiency at light loads due

to lower core losses and lower peak currents, but at high

load the smaller inductors are better because of lower

resistance. The inductor selection is generally based on

the ripple current which is typically set between 20% to

50% of the maximum load current. Cost, size, output ripple

and efficiency all play a part in the selection process.

The first step is to select the switching frequency. In this

case VOUT1 will be used at a nominal 270kHz.

For 15V input and 1.8V output, the typical on-time is:

TONtyp = VOUT/VIN/Freq

TONtyp = 444nsec.

The timing resistor RTON must be selected to provide

TONtyp:

RTON = (TONtyp – 35) × (VIN/(3.3 × VOUT) – 37

RTON = 976k.

We will use RTON = 1Meg.

Note that side2 will run typically 20% faster than side1, in

this case 320kHz.

Applications Information (continued)

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