LT3757 datasheet(17/28 Pages) AD

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LT8392

Rev. 0

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APPLICATIONS INFORMATION

Inductor Selection

The switching frequency and inductor selection are inter-

related in that higher switching frequencies allow the use

of smaller inductor and capacitor values. The inductor

value has a direct effect on ripple current. The highest cur-

rent ripple ∆IL% happens in the buck region at VIN(MAX),

and the lowest current ripple ∆IL% happens in the boost

region at VIN(MIN). For any given ripple allowance set by

customers, the minimum inductance can be calculated as:

LBUCK >

VOUT • VIN(MAX) − VOUT

(

)

f •IOUT(MAX) • ΔIL% • VIN(MAX)

LBOOST >

VIN(MIN)

2 • VOUT − VIN(MIN)

(

)

f •IOUT(MAX) • ΔIL% • VOUT

where:

ΔIL% =

ΔIL

IL(AVG)

f is switching frequency

VIN(MIN) is minimum input voltage

VIN(MAX) is maximum input voltage

VOUT is output voltage

IOUT(MAX) is maximum output current

Slope compensation provides stability in constant fre-

quency current mode control by preventing subharmonic

oscillations at certain duty cycles. The minimum induc-

tance required for stability when duty cycles are larger

than 50% can be calculated as:

10 • VOUT •RSENSE

For high efficiency, choose an inductor with low core loss,

such as ferrite. Also, the inductor should have low DC

resistance to reduce the I2R losses, and must be able to

handle the peak inductor current without saturating. To

minimize radiated noise, use a shielded inductor.

RSENSE Selection and Maximum Output Current

RSENSE is chosen based on the required output current.

The duty cycle independent maximum current sense

thresholds (50mV in peak-buck and 50mV in peak-boost)

set the maximum inductor peak current in buck region,

buck-boost region, and boost region.

In boost region, the lowest maximum average load cur-

rent happens at VIN(MIN) and can be calculated as:

IOUT(MAX_BOOST) =

50mV

RSENSE

−

ΔIL(BOOST)

⎛

⎝⎜

⎞

⎠⎟

•

VIN(MIN)

VOUT

where ∆IL(BOOST) is peak-to-peak inductor ripple current

in boost region and can be calculated as:

ΔIL(BOOST) =

VIN(MIN) • VOUT − VIN(MIN)

(

)

f •L • VOUT

In buck region, the lowest maximum average load current

happens at VIN(MAX) and can be calculated as:

IOUT(MAX_BUCK) =

50mV

RSENSE

−

ΔIL(BUCK)

⎛

⎝⎜

⎞

⎠⎟

where ∆IL(BUCK) is peak-to-peak inductor ripple current in

buck region and can be calculated as:

ΔIL(BUCK) =

VOUT • VIN(MAX) − VOUT

(

)

f •L • VIN(MAX)

The maximum current sense RSENSE in boost region is:

RSENSE(BOOST) =

2 • 50mV • VIN(MIN)

2 •IOUT(MAX) • VOUT + ΔIL(BOOST) • VIN(MIN)

The maximum current sense RSENSE in buck region is

RSENSE(BUCK) =

2 • 50mV

2 •IOUT(MAX) + ΔIL(BUCK)

The final RSENSE value should be lower than the calculated

RSENSE in both buck and boost regions. A 20% to 30%

margin is usually recommended. Always choose a low

ESL current sense resistor.

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