LT3757 datasheet(14/22 Pages) AD

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LT3935

Rev. 0

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Figure 4. Typical Average Conducted Emissions

APPLICATIONS INFORMATION

Setting Switching Frequency with the RT Pin

TheswitchingfrequencyoftheLT3935isprogrammedbya

resistor connected between the RT pin and GND. Values of

theRTresistorfrom45.3kupto523kprogramfrequencies

from 2MHz down to 200kHz as shown in Table 1. Higher

frequencies allow for smaller external components but

increase switching power losses and radiated EMI.

Table 1. RT Resistance Range

SWITCHING FREQUENCY

2.0MHz

45.3k

1.6MHz

59.0k

1.2MHz

80.6k

1.0MHz

97.6k

750kHz

133k

500kHz

205k

400kHz

255k

300kHz

348k

200kHz

523k

Synchronizing Switching Frequency

The switching frequency can also be synchronized to an

external clock connected to the SYNC/SPRD pin. The high

level of the external clock must be at least 1.4V, and the

frequency must be between 200kHz and 2MHz. The RT

resistor is still required in this case, and the resistance

should correspond to the frequency of the external clock.

If the external clock ever stops, the LT3935 will rely on

the RT resistor to set the frequency.

Enabling Spread Spectrum Frequency Modulation

Connecting SYNC/SPRD to INTVCC will enable spread

spectrum frequency modulation (SSFM). The switching

frequencywillvaryfromthefrequencysetbytheRTresistor

to 125% of that frequency. If neither synchronization nor

SSFM is required, connect SYNC/SPRD to GND.

As shown in Figure 4, enabling SSFM can significantly at-

tenuate the electromagnetic interference that the LT3935,

like all switching regulators, emits at the switching fre-

quency and its harmonics. This feature is designed to

help devices that include the LT3935 perform better in the

various standard industrial tests related to interference.

The attenuation varies depending on the chosen switching

frequency, the range of frequencies in which interference

is measured, and whether a test measures peak, quasi-

peak, or average emissions. The results of several other

such emission measurements are included with select

Typical Applications.

Understanding the Current Limit

The choice of switching frequency should be made know-

ing that, although the maximum LED current that can be

programmed with the CTRL pin is 4A, the inductor current

may exceed 4A when the frequency is high and the output

voltage is low as in a short-circuit. This is because there

is a minimum on-time for which the SW pin will be driven

high during each switching period. The inductor current

increases during this time, and if the frequency is high and

the output voltage low, there may not be enough off-time

remainingineachswitchingperiodfortheinductorcurrent

to decrease back to the level at which it started. In this

case, the net inductor current would increase with each

switching period regardless of the state of the CTRL pin.

To prevent large inductor currents that would damage

the LT3935, the high-side switch is not turned on until

the inductor current decreases to less than the inductor

“valley” limit (called the DA limit), which is approximately

6.4A. The peak inductor current may increase to 6.8A, but

theoff-timeandtheswitchingperiodareextendeduntilthe

inductor current reaches equilibrium as shown in Figure 5.

SSFM OFF

SSFM ON

FREQUENCY (MHz)

–20

–10

3935 F04

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