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LT3950

Rev. 0

For more information www.analog.com

Synchronizing the Switching Frequency

To synchronize LT3950 to an external clock, simply drive

the SYNC/SPRD pin with that clock signal. Choose the RT

resistor such that the system’s internal clock would be

within 10% of the external clock frequency. It is best to

have the RT programmed frequency as close as possible

to the external clock frequency.

Spread Spectrum Frequency Modulation

Like all switching converters, LT3950 does produce some

electromagnetic interference (EMI). Enabling spread spec-

trum frequency modulation can significantly attenuate this

interference. Conditions such as switching frequency and

printed circuit board (PCB) geometry affect the amount

of attenuation achievable with spread spectrum frequency

modulation. A variety of industry-standard tests exist to

quantify these effects.

To enable internal spread spectrum frequency modula-

tion, tie the SYNC/SPRD pin to INTVCC. The modulating

waveform is a triangle wave with steps at the positive and

negative peaks. The modulation frequency is around 9kHz,

and the switching frequency range is from 100-125% of

the programmed value. Typical LT3950 EMI results are

shown in Figure 5.

Maximum Duty Ratio

Since LT3950 uses a switch to connect an inductor from

VIN to ground, having a duty ratio of 100% would result in

zero current flowing to the load. To prevent this situation,

the part enforces a minimum off time. During this time,

irrespective of load or demand, the switch turns off and

allows the inductor current to flow into the load. The duty

ratio can, therefore, never reach 100%. The maximum

duty ratio varies with frequency. For the same minimum

off time, a higher frequency signal will have a smaller max-

imum duty ratio. At lower frequencies in boost configura-

tion and in Continuous Conduction Mode (CCM), the part

can reach a duty ratio of 95% (that is, VISP/VIN = 60/3, the

maximum range for the part). However at higher frequen-

cies, such as 2MHz, the part has a maximum duty ratio

of around 90%. Maximum duty ratio at any frequency in

CCM is given by the following relationship.

DMAX = 1– fSW • 50ns

APPLICATIONS INFORMATION

CISPR25 Conducted EMI Performance Current Method

CLASS 5 AVERAGE LIMIT

LT3950 SSFM ON

AMBIENT

FREQUENCY (MHz)

0.1

108

–60

–50

–40

–30

–20

–10

(a)

CISPR25 Conducted EMI Performance Voltage Method

CLASS 5 AVERAGE LIMIT

LT3950 SSFM ON

AMBIENT

FREQUENCY (MHz)

0.1

108

–20

–10

(b)

CISPR25 Radiated EMI Performance

CLASS 5 AVERAGE LIMIT

LT3950 SSFM ON

AMBIENT

FREQUENCY (MHz)

0.1

100

1000

–20

–10

(c)

Figure 5. LT3950 Typical EMI (VIN = 12V, VLED = 25V, ILED =

330mA, 2MHz)

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