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LT8386

Rev. 0

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APPLICATIONS INFORMATION

Setting Switching Frequency with the RT Pin

The switching frequency of the LT8386 is programmed by

a resistor connected between the RT pin and GND. Values

of the RT resistor from 45.3k up to 499k program frequen-

cies 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

2MHz

45.3k

1.6MHz

57.6k

1.2MHz

78.7k

1MHz

95.3k

400kHz

249k

210kHz

499k

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 1.6V, and the

low level must be below 0.8V. The frequency must be

between 200kHz and 2MHz. The RTresistor 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 LT8386 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

frequency will vary from the frequency set by the RT resis-

tor 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 signifi-

cantly attenuate the electromagnetic interference that

the LT8386, like all switching regulators, emits at the

switching frequency and its harmonics. This feature is

designed to help devices that include the LT8386 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

emission measurements are with select typical application

circuits.

Maximum Duty Cycle

The choice of switching frequency should be made know-

ing that the maximum VOUT voltage of a boost converter

is determined by the maximum duty cycle for a given VIN

voltage as shown in the following equation:

VOUT =

VIN

(1– D)

(1)

where D is the duty cycle of the boost converter defined

as the ratio of the on-time of the bottom power switch to

the total switching period. The maximum duty cycle for

a given switching frequency is determined by the mini-

mum off-time of the bottom power switch. The longest

minimum off-time of the LT8386 is 35ns, so the maxi-

mum duty cycle is 93% at 2MHz switching frequency.

Therefore, if an application requires higher duty cycle,

the switching frequency should be set lower to achieve

the demanded duty cycle.

Figure 4. Typical Conducted Peak EMI of the LT8386

with 2MHz Switching Frequency

–10

–20

FREQUENCY (MHz)

8386 F04

SSFM ON

SSFM OFF

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