LT8335 datasheet(11/20 Pages) AD

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LT8338

Rev. 0

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APPLICATIONS INFORMATION

While in Burst Mode operation (Figure 2), the current

limit of the bottom switch is approximately 300mA (as

shown in Switching Waveforms in Burst Mode Operation

in Typical Performance Characteristics), resulting in larger

output voltage ripple comparing to that in pulse-skipping

mode operation. Increasing the output capacitance will

decrease output ripple proportionally. As the load ramps

upward from zero, the switching frequency increases until

reaching the switching frequency programmed by the RT

resistor. The output load at which the LT8338 reaches the

programmed frequency varies based on input voltage,

output voltage, and inductor choice.

The LT8338 uses a constant-frequency architecture that

can be programmed over a 300kHz to 3MHz range with

a single external resistor from the RT pin to ground, as

shown in the Block Diagram.

Table 1 gives some specific examples of RT values for

specific switching frequencies.

Table 1. SW Frequency (fSW) vs RT Value

fSW (MHz)

RT (kΩ)

fSW (MHz)

RT (kΩ)

0.3

301

1.7

52.3

0.4

226

1.8

49.9

0.5

182

1.9

46.4

0.6

154

2.0

44.2

0.7

133

2.1

42.2

0.8

118

2.2

40.2

0.9

102

2.3

38.3

1.0

93.1

2.4

36.5

1.1

84.5

2.5

34.8

1.2

76.8

2.6

33.2

1.3

71.5

2.7

32.4

1.4

64.9

2.8

30.9

1.5

60.4

2.9

29.4

1.6

56.2

3.0

28.7

The operating frequency of the LT8338 can be synchro-

nized to an external clock source. By providing a clock

signal into the SYNC/MODE pin, the LT8338 operates

at the SYNC pulse frequency and automatically enters

pulse-skipping mode operation at light load. If this feature

is used, an RT resistor should be chosen to program a

switching frequency equal to, or slightly less than the

SYNC pulse frequency. For example, if the synchroniza-

tion signal is 500kHz or higher, the RT should be selected

for 500kHz. The slope compensation is set by the RT

value, while the minimum slope compensation required

to avoid subharmonic oscillations is established by the

inductor size, input voltage, and output voltage. Since

the synchronization frequency will not change the slope

of the inductor current waveform, if the inductor is large

enough to avoid subharmonic oscillation at the frequency

set by RT, then the slope compensation will be sufficient

for all synchronization frequencies.

Figure 2. Burst Mode Operation Waveforms

For some applications it is desirable for the LT8338 to

operate in pulse-skipping mode. Pulse-skipping mode

operation offers two major differences from Burst Mode

operation. First the clock stays awake at all times and all

switching cycles are aligned to the clock. In this mode

much of the internal circuitry is awake at all times, increas-

ing quiescent current to thousand μA (compared to 6μA

quiescent current in Burst Mode operation). Secondly

pulse-skipping mode operation exhibits lower output

ripple as well as lower audio noise and RF interference.

Operating Frequency and Synchronization

The choice of operating 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.

2µs/DIV

8338 F02

VSW

20V/DIV

200mA/DIV

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