Data Sheet

LT8349

analog.com

Rev B

17 of 29

In order to achieve higher light load efficiency, more energy should be delivered to the output during the single pulses

in Burst Mode operation such that the LT8349 can stay in sleep mode longer between each pulse. This can be

achieved by selecting a higher IBURST current by the ISET pin (Pin Functions section). The light load efficiency can be

further increased by using larger value inductors. The tradeoff is that while more energy is delivered to the output by

the single pulses, the output voltage ripple will be larger at light load. However, the output voltage ripple can be

reduced proportionally by increasing the output capacitance. When adjusting inductor and/or output capacitor

values, a careful evaluation of system stability should be made to ensure adequate design margin. Larger inductors

and/or output capacitors also increase cost and solution sizes.

Figure 24c shows the IL1 and IL2 currents when Forced Continuous operation (FCM) is selected by SYNC/MODE pin. As

shown in Figure 24c, at light load the IL2 is kept to 0A and the IL1 is allowed to go negative so that the regulator can

switch at the programmed frequency all the way down to zero output current. This has the advantage of maintaining

the programmed switching frequency across the entire load range so that the switch harmonics and EMI are

consistent and predictable. The disadvantage of FCM is that the light load efficiency will be lower compared to Burst

Mode operation

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Switching Frequency and Synchronization

The choice of switching frequency is a trade-off between efficiency and component size. Low frequency operation

improves efficiency by reducing the power switches’ switching losses and gate drive current. However, lower

frequency operation requires a physically larger inductor. The LT8349 uses a constant-frequency architecture that

can be programmed over a 300kHz to 4MHz range with a single external resistor from the RT pin to ground, as shown

in Block Diagram. A table for selecting the value of RT for a given switching frequency is shown in Table 4. Figure 25

shows the RT Value vs Switching Frequency curve.

Figure 25. RTValue vs Switching Frequency

Table 4. SW Frequency (fSW) vs RT Value

fSW (MHz)

RT (kΩ)

fSW(MHz)

RT (kΩ)

0.3

464

2.2

48.7

0.4

348

2.3

45.2

0.5

274

2.4

43.2

0.6

221

2.5

40.2

0.7

187

2.6

38.3

0.8

162

2.7

36.5

0.9

143

2.8

34.8

SWITCHING FREQUENCY (MHz)

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

10

100

500