LT8331 datasheet(10/30 Pages) LINER

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LT8331

8331fa

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APPLICATIONS INFORMATION

No additional components or loading is allowed on this

pin. The INTVCC rising threshold (to allow soft-start and

switching) is typically 2.6V. The INTVCC falling threshold

(to stop switching and reset soft-start) is typically 2.5V.

To improve efficiency across all loads, the majority of

INTVCC current can be drawn from the BIAS pin (4.4V ≤

BIAS ≤ VIN – 0.4V) instead of the VIN pin. For flyback or

SEPIC applications with VIN often greater than VOUT, the

BIAS pin can be directly connected to VOUT. If the BIAS

pin is connected to a supply other than VOUT, be sure to

bypass the pin with a local ceramic capacitor.

Programming Switching Frequency

The LT8331 uses a constant frequency PWM architecture

that can be programmed to switch from 100kHz to 500kHz

by using a resistor tied from the RT pin to ground. A table

showing the necessary RT value for a desired switching

frequency is in Table 1.

The RT resistor required for a desired switching frequency

can be calculated using:

RT =

32.85

fSW

– 9.5

where RT is in kΩ and fSW is the desired switching fre-

quency in MHz.

Table 1. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.1

324

0.2

154

0.3

100

0.4

73.2

0.45

63.4

0.5

56.2

Synchronization and Mode Selection

To select low ripple Burst Mode operation, tie the SYNC/

MODE pin below 0.6V (this can be ground or a logic low

output). To synchronize the LT8331 oscillator to an exter-

nal frequency connect a square wave (with 20% to 80%

duty cycle) to the SYNC pin. The square wave amplitude

should have valleys that are below 0.6V and peaks above

2.4V (up to 6V).

The LT8331 will not enter Burst Mode operation at low

output loads while synchronized to an external clock, but

instead will pulse skip to maintain regulation. The LT8331

may be synchronized over a 100kHz to 625kHz range. The

RT resistor should be chosen to set the LT8331 switching

frequency equal to or below the lowest synchronization

input. For example, if the synchronization signal will be

500kHz and higher, the RT should be selected for 500kHz.

For some applications it is desirable for the LT8331 to

operate in pulse-skipping mode, offering two major differ-

ences from Burst Mode operation. Firstly, the clock stays

awake at all times and all switching cycles are aligned to

theclock.Secondly,thefullswitchingfrequencyisreached

at lower output load than in Burst Mode operation. These

twodifferencescomeattheexpenseofincreasedquiescent

current. To enable pulse-skipping mode, tie the SYNC pin

above 2.4V (this can be INTVCC or a logic high output).

DUTY CYCLE CONSIDERATION

The LT8331 minimum on-time, minimum off-time and

switching frequency (fOSC) define the allowable minimum

and maximum duty cycles of the converter (see Minimum

On-Time, Minimum Off-Time, and Switching Frequency

in the Electrical Characteristics table).

Minimum Allowable Duty Cycle =

Minimum On-Time(MAX) • fOSC(MAX)

Maximum Allowable Duty Cycle =

1 – Minimum Off-Time(MAX) • fOSC(MAX)

The required switch duty cycle range for a Boost converter

operating in continuous conduction mode (CCM) can be

calculated as:

DMIN = 1 –

VIN(MAX)

(VOUT + VD)

DMAX = 1 –

VIN(MIN)

(VOUT + VD)

where VD is the diode forward voltage drop. If the above

duty cycle calculations for a given application violate

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