LTC7803 datasheet(23/42 Pages) AD

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LTC7811

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 3. Setting the Operating Frequency

FREQ PIN RESISTOR (Ω)

10k

100k

500k

100k

10M

7811 F03b

Selecting the Light-Load Operating Mode

The LTC7811 can be set to enter high efficiency Burst

Mode operation, constant frequency pulse-skipping mode

or forced continuous conduction mode at light load cur-

rents. To select Burst Mode operation, tie the MODE to

ground. To select forced continuous operation, tie the

MODE pin to INTVCC. To select pulse-skipping mode, tie

the MODE pin to INTVCC through a 100k resistor. An inter-

nal 100k resistor from the MODE pin to ground selects

Burst Mode if the pin is floating. When synchronized to

an external clock through the PLLIN/SPREAD pin, the

LTC7811 operates in pulse skipping mode if it is selected,

or in forced continuous mode otherwise. Table 2 sum-

marizes the use of the MODE pin to select the light load

operating mode.

Table 2.

MODE PIN

LIGHT-LOAD OPERATING

MODE

MODE WHEN

SYNCHRONIZED

0V or Floating

Burst Mode

Forced Continuous

100k to INTVCC

Pulse-Skipping

INTVCC

Forced Continuous

In general, the requirements of each application will dictate

the appropriate choice for light-load operating mode. In

Burst Mode operation, the inductor current is not allowed

to reverse. The reverse current comparator turns off the

bottom MOSFET just before the inductor current reaches

zero, preventing it from reversing and going negative.

Thus, the regulator operates in discontinuous operation.

In addition, when the load current is very light, the induc-

tor current will begin bursting at frequencies lower than

the switching frequency, and enter a low current sleep

mode when not switching. As a result, Burst Mode opera-

tion has the highest possible efficiency at light load.

77.5% at 2.25MHz

FREQUENCY (Hz)

100k

100

7811 F03a

(a) Relationship Between Oscillator Frequency and

Boost Channel Maximum Duty Cycle

(b) Relationship Between Oscillator Frequency and

Resistor Value at the FREQ Pin

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