Data Sheet

LT8349

analog.com

Rev B

19 of 29

Spread Spectrum Frequency Modulation

The LT8349 features spread spectrum frequency modulation to further reduce EMI emissions. The user can select

spread spectrum frequency modulation with Burst Mode operation by connecting the SYNC/MODE pin to ground

through a 50k resistor or spread spectrum frequency modulation with FCM operation by connecting the SYNC/MODE

pin to VIN pin. When spectrum frequency modulation is selected, a stepped triangular frequency modulation is used

to vary the internal oscillator frequency between the value programmed by the RTresistor to approximately 25% (at

Fsw_set = 2MHz) higher than that value. Figure 28 shows spread spectrum depth (typ) variation with switching

frequency. The modulation frequency is approximately 0.45% of the switching frequency. For example, when the

LT8349 is programmed to 2MHz, and spread spectrum frequency modulation is selected, the oscillator frequency

varies from 2MHz to 2.5MHz at a 9kHz rate (see Oscillator Frequency with Spread Spectrum Modulation curve in the

Typical Performance Characteristics section). When operating at light load, the spread spectrum frequency

modulation is more effective in FCM mode than in Burst Mode operation, due to the fact that FCM operation

maintains the programmed switching frequency across entire load current range.

Figure 28. SSFM Range as Percentage of Fsw (typ) vs. Switching Frequency

FB Resistor Network and the Quiescent Current at No Load

The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor

values according to the following equation:

R1 = R2 • (

VOUT

1V

− 1)

Reference designators refer to Block Diagram. The 1% resistors are recommended to maintain output voltage

accuracy. If low input quiescent current and good light-load efficiency are desired, use large resistor values for the

FB resistor divider. The current flowing in the divider acts as a load current and will increase the no load input current

to the converter. The LT8349 converter Burst Mode quiescent current at no load can be estimated using the following

Equation:

IQ ≈ 15μA + (

VOUT

R1 + R2

+ 1μA) •

VOUT

VIN

• 3

where 15µA is the VIN pin quiescent current of the LT8349, and the second term is the current drawn by the feedback

divider and VOUT pin (1µA) reflected to the input of the boost operating. For a 3.7V input, 6V output boost converter

with R1 = 1MΩ and R2 = 200kΩ, it can be calculated that the LT8349 converter draws approximately 45µA from the

supply at no load.

SWITCHING FREQUENCY (MHz)

0.3

1

4

0

5

10

15

20

25

30

35