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LT8390

8390fa

For more information www.linear.com/LT8390

APPLICATIONS INFORMATION

Frequency Synchronization

The LT8390 switching frequency can be synchronized to

an external clock using the SYNC/SPRD pin. Driving the

SYNC/SPRD with a 50% duty cycle waveform is always a

good choice, otherwise maintain the duty cycle between

10%and90%.Duetotheuseofaphase-lockedloop(PLL)

inside, there is no restriction between the synchronization

frequency and the internal oscillator frequency. The rising

edge of the synchronization clock represents the begin-

ning of a switching cycle, turning on switches A and C,

or switches A and D.

Inductor Selection

The switching frequency and inductor selection are inter-

relatedinthathigherswitchingfrequenciesallowtheuseof

smaller inductor and capacitor values. The inductor value

has a direct effect on ripple current. The highest current

ripple∆IL%happensinthebuckregionatVIN(MAX),andthe

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) • ∆IL% • VIN(MAX)

LBOOST >

VIN(MIN)

2 • V

OUT − VIN(MIN)

(

)

f •IOUT(MAX) • ∆IL% • VOUT

where:

∆IL% =

∆IL

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

oscillationsatcertaindutycycles.Theminimuminductance

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 current

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

IOUT(MAX_BOOST) =

50mV

RSENSE

−

∆IL(BOOST)











 •

VIN(MIN)

VOUT

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

in boost region and can be calculated as:

∆IL(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

−

∆IL(BUCK)













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

buck region and can be calculated as:

∆IL(BUCK) =

VOUT • VIN(MAX) − VOUT

(

)

f •L • VIN(MAX)

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