LTC1430AIGN datasheet(11/24 Pages) LINER

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LTC1430A

APPLICATI

I FOR ATIO

are only 1.1W per device or less — large TO-220 packages

and heat sinks are not necessarily required in high effi-

ciency applications. Siliconix Si4410DY (in SO-8) and

Motorola MTD20N03HL (in DPAK) are two small, surface

mount devices with RON values of 0.03Ω or below with 5V

of gate drive; both work well in LTC1430A circuits with up

to 10A output current. A higher PMAX value will generally

decrease MOSFET cost and circuit efficiency and increase

MOSFET heat sink requirements.

Inductor

The inductor is often the largest component in an LTC1430A

design and should be chosen carefully. Inductor value and

type should be chosen based on output slew rate require-

ments and expected peak current. Inductor value is prima-

rily controlled by the required current slew rate. The

maximum rate of rise of the current in the inductor is set

by its value, the input-to-output voltage differential and the

maximum duty cycle of the LTC1430A. In a typical 5V to

3.3V application, the maximum rise time will be:

90%

(VIN – VOUT)

AMPS

SECOND

1.53A

µs

where L is the inductor value in

µH. A 2µH inductor would

have a 0.76A/

µs rise time in this application, resulting in a

6.5

µsdelayinrespondingtoa5Aloadcurrentstep.During

this 6.5

µs,thedifferencebetweentheinductorcurrentand

the output current must be made up by the output capaci-

tor, causing a temporary droop at the output. To minimize

this effect, the inductor value should usually be in the 1

µH

to 5

µH range for most typical 5V to 2.xV-3.xV LTC1430A

circuits. Different combinations of input and output volt-

ages and expected loads may require different values.

Once the required value is known, the inductor core type

can be chosen based on peak current and efficiency

requirements. Peak current in the inductor will be equal to

the maximum output load current added to half the peak-

to- peak inductor ripple current. Ripple current is set by the

inductor value, the input and output voltage and the

operating frequency. If the efficiency is high and can be

approximately equal to 1, the ripple current is approxi-

mately equal to:

∆I =

(VIN – VOUT)

(fOSC)(L)

DC =

VOUT

VIN

fOSC = LTC1430A oscillator frequency

L = inductor value

Solving this equation with our typical 5V to 3.3V applica-

tion, we get:

= 2.8AP–P

(1.7)(0.66)

(200kHz)(2

µH)

Peak inductor current at 10A load:

= 11.4A

10A +

2.8A

The inductor core must be adequate to withstand this peak

current without saturating, and the copper resistance in

the winding should be kept as low as possible to minimize

resistive power loss. Note that the current may rise above

this maximum level in circuits under current limit or under

fault conditions in unlimited circuits; the inductor should

be sized to withstand this additional current.

Input and Output Capacitors

A typical LTC1430A design puts significant demands on

both the input and output capacitors. Under normal steady

load operation, a buck converter like the LTC1430A draws

square waves of current from the input supply at the

switching frequency, with the peak value equal to the

output current and the minimum value near zero. Most of

this current must come from the input bypass capacitor,

since few raw supplies can provide the current slew rate to

feed such a load directly. The resulting RMS current flow

in the input capacitor will heat it up, causing premature

capacitor failure in extreme cases. Maximum RMS current

occurs with 50% PWM duty cycle, giving an RMS current

value equal to IOUT/2. A low ESR input capacitor with an

adequate ripple current rating must be used to ensure

reliable operation. Note that capacitor manufacturers’

ripple current ratings are often based on only 2000 hours

(3 months) lifetime; further derating of the input capacitor

ripple current beyond the manufacturer’s specification is

recommended to extend the useful life of the circuit.

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