LT8331 datasheet(13/24 Pages) AD

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LT8337/LT8337-1

Rev. 0

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While in Burst Mode operation the bottom switch peak

current is approximately 1.2A as shown in the Switching

Waveforms in Burst Mode operation curve in the Typical

Performance Characteristics section. This behavior

results in larger output voltage ripple compared to that

in pulse-skipping mode operation which has lower bot-

tom switch peak current. However, the output voltage

ripple can be reduced proportionally by increasing the

output capacitance. When adjusting output capacitance,

a careful evaluation of system stability should be made

to ensure adequate design margin. As the load ramps

upward from zero, the switching frequency keeps increas-

ing until reaching the switching frequency programmed

by the resistor at the RT pin. The output load at which the

LT8337/LT8337-1 reaches the programmed frequency

varies based on input voltage, output voltage, and induc-

tor choice.

Pulse-skipping mode operation offers two major dif-

ferences from Burst Mode operation. First, the internal

clock stays awake at all times and all switching cycles are

aligned to the clock. In this mode the internal circuitry is

awake at all times, increasing quiescent current to several

hundred μA compared to the 5μA of VIN pin quiescent cur-

rent in Burst Mode operation. Second, as the load ramps

upward from zero, the switching frequency programmed

by the resistor at the RT pin is reached at a lower output

load than in Burst Mode operation, therefore, pulse-skip-

ping mode operation exhibits lower output ripple as well

as lower audio noise and RF interference.

Switching Frequency and Synchronization

The choice of switching frequency is a trade-off between

efficiency and component size. Low frequency opera-

tion improves efficiency by reducing the power switches’

switching losses and gate drive current. However, lower

frequency operation requires a physically larger inductor.

The LT8337/LT8337-1 uses a constant-frequency archi-

tecture that can be programmed over a 300kHz to 3MHz

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 1. Figure 1 shows the RT Value vs Switching

Frequency curve.

Table 1. SW Frequency (fSW) vs RT Value

fSW (MHz)

RT (kΩ)

fSW (MHz)

RT (kΩ)

0.3

357

1.7

57.6

0.4

267

1.8

53.6

0.5

210

1.9

51.1

0.6

174

2.0*

47.5

0.7

147

2.1

45.2

0.8

127

2.2

43.2

0.9

113

2.3

40.2

1.0

102

2.4

39.2

1.1

90.9

2.5

37.4

1.2

84.5

2.6

35.7

1.3

76.8

2.7

34.0

1.4

71.5

2.8

32.4

1.5

64.9

2.9

30.9

1.6

61.9

3.0

30.1

* Programming 2MHz will ensure fSW stays above 1.85MHz (out of the

AM band).

The operating frequency of the LT8337/LT8337-1 can

be synchronized to an external clock source with 100ns

minimum pulse width. By providing a digital clock signal

to the SYNC/MODE pin, the IC operates at the SYNC pulse

frequency and automatically enters pulse-skipping mode

operation at light load. If this feature is used, an RT resis-

tor should be chosen to program a switching frequency

as close as possible to the SYNC pulse frequency.

APPLICATIONS INFORMATION

Figure 1. RT Value vs Switching Frequency

SWITCHING FREQUENCY (kHz)

500

1000 1500 2000 2500 3000

10k

100k

83371 F01

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