LT3755 datasheet(19/32 Pages) AD

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LT8393

Rev. A

For more information www.analog.com

Spread Spectrum Frequency Modulation

Switching regulators can be particularly troublesome for

applications where electromagnetic interference (EMI) is

a concern. To improve the EMI performance, the LT8393

implementsatrianglespreadspectrumfrequencymodula-

tion scheme. With the SYNC/SPRD pin tied to INTVCC, the

LT8393startstospreaditsswitchingfrequency25%above

the internal oscillator frequency. Figure 9 and Figure 10

show the noise spectrum comparison of the front page

application with spread spectrum enabled.

APPLICATIONS INFORMATION

Figure 9. CISPR 25 Average Conducted EMI

Figure 10. CISPR 25 Peak Conducted EMI

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)

•ILED(MAX)• ΔIL %•VIN(MAX)

LBOOST >

VIN(MIN)2•(VOUT− VIN(MIN))

•ILED(MAX)•ΔIL %•VOUT2

where:

f is switching frequency

∆IL% is allowable inductor current ripple

VIN(MIN) is minimum input voltage

VIN(MAX) is maximum input voltage

VOUT is output voltage

ILED(MAX) is maximum LED current

Slope compensation provides stability in constant fre-

quency current mode control by preventing subharmonic

oscillationsatcertaindutycycles.Theminimuminductance

required for stability can be calculated as:

•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.

FREQUENCY (MHz)

–20

–10

0.1

100

8393 F09

NOISE FLOOR

SSFM ON WITH FILTER

CLASS 5 LIMITS

FREQUENCY (MHz)

–20

–10

0.1

100

8393 F10

NOISE FLOOR

CLASS 5 LIMITS

SSFM ON WITH FILTER

Frequency Synchronization

The LT8393 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

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