Data Sheet

LT8342

analog.com

Rev. 0

23 of 36

Table 7. Switching Frequency (fsw) vs RT Value

fSW (MHz)

RT (kΩ)

fSW (MHz)

RT (kΩ)

0.3

110

1.7

18.2

0.4

84.5

1.8

16.9

0.5

68.1

1.9

15.8

0.6

56.2

2.0

15

0.7

48.7

2.1

14.3

0.8

42.2

2.2

13.7

0.9

36.5

2.3

13

1.0

31.6

2.4

12.4

1.1

28.7

2.5

11.8

1.2

26.1

2.6

11.3

1.3

24.3

2.7

10.7

1.4

22.1

2.8

10.2

1.5

20.5

2.9

9.76

1.6

19.1

3.0

9.53

Spread Spectrum Frequency Modulation

The LT8342 features SSFM to further reduce electromagnetic emissions. The user can select SSFM with Burst Mode

operation by connecting the SYNC/MODE pin to ground through a 100kΩ resistor. The user can also select SSFM

with pulse-skipping operation by connecting the SYNC/MODE pin to INTVCC. When SSFM is selected, a stepped

triangular frequency modulation is used to vary the internal oscillator frequency between the value programmed

by the RT resistor to approximately 13% higher than that value. The modulation frequency is approximately 0.45%

of the switching frequency. For example, when the LT8342 is programmed to 2MHz, and spread spectrum

frequency modulation is selected, the oscillator frequency varies from 2MHz to 2.3MHz at a 9kHz rate (see Figure 12,

Oscillator Frequency with Spread Spectrum Modulation in the Typical Performance Characteristics section).

VIN to VOUT PassThru Mode Operation

In automotive PreBoost applications, the input voltage of the boost converter is normally above the regulated

output voltage programmed by the feedback resistor network. Under this condition, the LT8342 enters PassThru

mode operation, where the synchronous power switch M2 (See Block Diagram) is kept on continuously, and the

power switch M1 is kept off continuously. The voltage across the boost capacitor (CBST) is constantly monitored.

When it drops below 2.9V, an internal charge pump circuit is turned on to charge the boost capacitor voltage to

3.2V and then turned off, maintaining enough gate drive voltage on M2.

In PassThru mode, the LT8342 intelligently enters a low quiescent current state regardless of the SYNC/MODE pin's

configuration. Input quiescent current can be as low as 26.5μA with the input-disconnect FET (MEXT), or 12μA

without MEXT. Note that if the input voltage exceeds 110% of the target output voltage, the PG pin will pull low to

indicate that VOUT is out of regulation. If PG is externally pulled up, the current through the pull-up resistor can be

more significant than the quiescent current of LT8342. To further reduce the power consumption in PassThru, one