LT3952 datasheet(21/32 Pages) LINER

21 / 32 page

LT3952

3952fb

For more information www.linear.com/LT3952

OPERATION

The overall switching period is defined by the sum of

two parts: a fixed 50ns off-time that defines DMAX, and a

variable time defined by the RT current. To determine the

proper RT value for a desired switching frequency, using

the following equation:

RT kΩ

()=

88.9

fSW (MHz)1.13

For example, a 2MHz switching frequency results in a

desired RT resistance of 40.6k, which has 40.2k as the

closest standard value.

Table 1 provides some commonly used values.

Table 1. Switching Frequency vs RT Value

fSW (MHz)

RT (k)

3.0

25.5

2.0

40.2

1.0

90.9

0.4

243

0.215

470

Frequency Synchronization and Spread Spectrum

The LT3952 SYNC pin acts as both an external clock input

for frequency synchronization and the enable signal for

internal spread spectrum feature. Tie the SYNC/SPRD pin

low for fixed-frequency internal clock, tie to INTVCC for

spread-spectrum internal clock, or drive with an external

clock for frequency synchronization with no internal

spread spectrum.

When synchronizing to an external clock, the RT resis-

tor should be chosen to program a switching frequency

20% lower than the external clock frequency. Even when

synchronizing, a soft-start cycle will first start the oscil-

lator in frequency foldback to minimize inrush current. As

the soft-start cycle nears completion, the device will then

transition to the external frequency.

When the SYNC/SPRD pin is tied high for greater than

32 clock cycles, the device will enable spread-spectrum

clocking for EMI reduction. By continuously varying the

oscillator frequency, spread spectrum distributes the EMI

power generated during the switching cycle over a group

of frequencies rather than concentrating it at a single

frequency. Therefore, the measured EMI power at any

single frequency is reduced compared to that of fixed-

frequency operation.

Inspread-spectrummode,theoscillatorfrequencyisvaried

in a pseudorandom manner from the nominal frequency

to 31% above nominal in 1% steps. This unidirectional

adjustment allows LT3952 to avoid a sensitive band in the

system simply by programming the nominal frequency

slightly above it. The proportional step size allows the user

to easily determine RT value for their specified EMI test

bin size, and the pseudorandom method provides tone

suppression from the frequency variation itself.

LT3952 Datasheet(PDF) 21 Page - Linear Technology

LT3952 Datasheet(HTML) 21 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL