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LT8645S-2

Rev. A

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APPLICATIONS INFORMATION

In order to achieve higher light load efficiency, more

energy must be delivered to the output during the sin-

gle small pulses in Burst Mode operation such that the

LT8645S-2 can stay in sleep mode longer between each

pulse. This can be achieved by using a larger value induc-

tor (i.e., 4.7µH), and should be considered independent

of switching 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. See curve in Typical Performance

Characteristics.

While in Burst Mode operation the current limit of the top

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

resulting in low output voltage ripple. Increasing the

output capacitance will decrease output ripple propor-

tionally. As load ramps upward from zero the switch-

ing frequency will increase but only up to the switching

frequency programmed by the resistor at the RT pin as

shown in Figure 2a.

The output load at which the LT8645S-2 reaches the pro-

grammed 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-2 to

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

ences 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 current 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-2 features spread spectrum operation 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 programmed

by RT to approximately 20% higher than that value. The

modulation frequency is approximately 3kHz. For exam-

ple, when the LT8645S-2 is programmed to 2MHz, the

frequency will vary from 2MHz to 2.4MHz at a 3kHz rate.

When spread spectrum operation is selected, Burst Mode

operation is disabled, and the part will run in pulse-skip-

ping mode.

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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