LT8646SEVPBF datasheet(18/30 Pages) AD

18 / 30 page

LT8645S/LT8646S

Rev. B

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APPLICATIONS INFORMATION

loads, the current in the feedback resistor divider must

be minimized as it appears to the output as load current.

Inordertoachievehigherlightloadefficiency,moreenergy

must be delivered to the output during the single small

pulses in Burst Mode operation such that the LT8645S/

LT8646Scanstayinsleepmodelongerbetweeneachpulse.

This can be achieved by using a larger value inductor (i.e.,

4.7µH), and should be considered independent of switch-

ing frequency when choosing an inductor. For example,

while a lower inductor value would typically be used for

a high switching frequency application, if high light load

efficiency is desired, a higher inductor value should be

chosen.SeecurveinTypicalPerformanceCharacteristics.

While in Burst Mode operation the current limit of the top

switch is approximately 1.25A (as shown in Figure 3),

resultinginlowoutputvoltageripple.Increasingtheoutput

capacitance will decrease output ripple proportionally. As

load ramps upward from zero the switching frequency

will increase but only up to the switching frequency pro-

grammedbytheresistorattheRTpinasshowninFigure 2a.

The output load at which the LT8645S/LT8646S reaches

the programmed frequency varies based on input voltage,

output voltage, and inductor choice. To select low ripple

Burst Mode operation, tie the SYNC/MODE pin below 0.4V

(this can be ground or a logic low output).

Pulse-Skipping Mode

For some applications it is desirable for the LT8645S/

LT8646S to operate in pulse-skipping mode, offering two

major differences from Burst Mode operation. First is 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, increasing quiescent cur-

rent to several hundred µA. Second is that full switching

frequency is reached at lower output load than in Burst

Mode operation (see Figure 2b). To enable pulse-skipping

mode, float the SYNC/MODE pin. Leakage current in this

pin should be <1µA. See Block Diagram for internal pull-up

and pull-down resistance.

Spread Spectrum Mode

The LT8645S/LT8646S features spread spectrum opera-

tion to further reduce EMI emissions. To enable spread

spectrum operation, the SYNC/MODE pin should be tied

high to INTVCC (~3.4V) or an external supply of 3V to 4V.

In this mode, triangular frequency modulation is used

to vary the switching frequency between the value pro-

grammed by RT to approximately 20% higher than that

value. The modulation frequency is approximately 3kHz.

For example, when the LT8645S/LT8646S is programmed

to 2MHz, the frequency will vary from 2MHz to 2.4MHz at

Figure 3. Burst Mode Operation

Switching Waveforms,

Burst Mode Operation

FRONT PAGE APPLICATION

12VIN TO 5VOUT AT 10mA

VSYNC = 0V

10µs/DIV

VSW

5V/DIV

500mA/DIV

8645S F03

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