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LT3433

3433ia

Ripple current:

∆=

() −

()

−

LP P

OUT

()

•

Inductor Selection

The primary criterion for inductor value selection in LT3433

applications is the ripple current created in that inductor.

Design considerations for ripple current are the amount of

output ripple and the ability of the internal slope compen-

sation waveform to prevent current mode instability.

The LT3433 maximizes available dynamic range using a

slope compensation generator that generates a continu-

ously increasing slope as duty cycle increases. The slope

compensation waveform is calibrated at 80% duty cycle to

compensate for ripple currents up to 12.5% of IMAX, or

~ 60mA.

Ripple current can be calculated as:

∆=

() −

()

−

LP P

OUT

()

•

This relation can be used to determine minimum induc-

tance sizes for various values of VOUT using the DC = 80%

calibration:

LMIN = (VOUT + 1.5V) • (1 – 0.8) 60mA • 200kHz)

VOUT

LMIN

µH

108

µH

175

µH

12V

225

µH

APPLICATIO S I FOR ATIO

Discontinuous operation occurs when the ripple current in

the inductor is greater than twice the load current (ILOAD)

in buck mode, or greater than ILOAD/(1 – DC) during

bridged mode. Current mode instability is not a concern

during discontinuous operation so inductor values smaller

than LMIN can be used. If such a small inductor is used,

however, it must be assured that the converter never

enters continuous operation at duty cycles greater than

50% to prevent current mode instability.

Design Example

VIN(MIN) = 4V, VOUT = 5V, L = 150µH

Using VF = 0.75V yields:

DC = (VOUT + 2VF)/(VOUT + VIN + 2VF – VSWH – VSWL)

= (5V + 1.5V)/(4V + 5V + 1.5V – 0.6V – 0.5V)

= 0.69

∆IL = (VOUT + 2VF) • (1 – DC) • (L • f0)–1

= (5V + 1.5V) • (1 – 0.69) • (150

µH • 200kHz)–1

= 67mA

ILOAD(MAX) = ISW(MAX) • (1 – 1.1 • DC)

= [0.5A – (1/2 • 0.07)](1 – 1.1 • 0.69) = 0.112A

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