LTC7891 datasheet(21/36 Pages) AD

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Data Sheet

LTC7891

APPLICATIONS INFORMATION

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Rev. 0 | 21 of 36

SELECTING THE LIGHT LOAD OPERATING

MODE

The LTC7891 can be set to enter high efficiency Burst Mode opera-

tion, constant frequency pulse skipping mode, or forced continuous

conduction mode at light load currents. To select Burst Mode

operation, tie the MODE pin to GND. To select forced continuous

operation, tie the MODE pin to INTVCC. To select pulse skipping

mode, tie the MODE pin to INTVCC through a 100 kΩ resistor. An

internal 100 kΩ resistor from the MODE pin to GND selects Burst

Mode if the pin is floating. When synchronized to an external clock

through the PLLIN/SPREAD pin, the LTC7891 operates in pulse

skipping mode if it is selected. Otherwise, the LTC7891 operates in

forced continuous mode. Table 5 summarizes the use of the MODE

pin to select the light load operating mode.

Table 5. Using the MODE Pin to Select Light Load Operating Mode

MODE Pin

Light Load Operating Mode Mode When Synchronized

0 V or Floating

Burst Mode

Forced continuous

100 kΩ to INTVCC Pulse skipping

Pulse skipping

INTVCC

Forced continuous

The requirements of each application 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 FET before the inductor current reaches zero,

preventing it from reversing and going negative. Therefore, the

regulator operates in discontinuous operation. In addition, when

the load current is light, the inductor current begins bursting at

frequencies lower than the switching frequency and enters a low

current sleep mode when not switching. As a result, Burst Mode

operation has the highest possible efficiency at light loads.

In forced continuous mode, the inductor current is allowed to re-

verse at light loads and switches at the same frequency regardless

of the load. In this mode, the efficiency at light loads is considerably

lower than in Burst Mode operation. However, continuous operation

has the advantage of lower output voltage ripple and less interfer-

ence to audio circuitry. In forced continuous mode, the output ripple

is independent of the load current.

In pulse skipping mode, constant frequency operation is maintained

down to approximately 1% of the designed maximum output cur-

rent. At very light loads, the PWM comparator can remain tripped

for several cycles and force the top FET to remain off for the same

number of cycles (that is, skipping pulses). The inductor current is

not allowed to reverse (discontinuous operation). This mode, like

forced continuous operation, exhibits low output ripple as well as

low audio noise and reduced RF interference compared to Burst

Mode operation. Pulse skipping mode provides higher light load

efficiency than forced continuous mode, but not nearly as high

as Burst Mode operation. Consequently, pulse skipping mode rep-

resents a compromise between light load efficiency, output ripple,

and EMI.

In some applications, it may be desirable to change the light load

operating mode based on the conditions present in the system. For

example, if a system is inactive, the user can select high efficiency

Burst Mode operation by keeping the MODE pin set to 0 V. When

the system wakes, the user can send an external clock to PLLIN/

SPREAD, or tie MODE to INTVCC to switch to low noise forced

continuous mode. These on-the-fly mode changes can allow an

individual application to benefit from the advantages of each light

load operating mode.

DEAD TIME CONTROL (DTCA AND DTCB

PINS)

The dead time delays of the LTC7891 can be adjusted from near

zero to 60 ns by configuring the DTCA pin and DTCB pin. Refer to

Figure 40, Figure 41, and Figure 42, which show the TG minus SW,

BG, and SW waveforms for each DTCx pin setting. In the DTCx

Pins Tied to GND (Adaptive Dead Time Control), DTCx Pins Tied

to INTVCC (Smart Near Zero Dead Time Control), and DTCx Pins

Connected with a Resistor to GND sections, TG represents the

voltage sensed at the top FET gate (the threshold for TG falling is

sensed at the TGUP pin), and BG represents the voltage sensed

at the bottom FET gate (the thresholds for BG rising and falling

are sensed at the BGDN and BGUP pins, respectively). The SW

waveforms represent operation in continuous conduction mode with

positive inductor current. The DTCA pin programs the dead time

associated with the bottom FET turning off and the top FET turning

on (SW transitioning from low to high). The DTCB pin programs the

dead time associated with top FET turning off and the bottom FET

turning on (SW transitioning from high to low).

DTCx Pins Tied to GND (Adaptive Dead Time

Control)

Tying the DTCA pin and DTCB pin to GND programs adaptive dead

time control. In adaptive control (see Figure 40), the dead time is

measured between one FET turning off and the other FET turning

on. Tying the DTCA pin to GND fixes the delay between BG going

low and TG minus SW going high to approximately 20 ns. Tying the

DTCB pin to GND fixes the delay between TG minus SW falling and

BG rising to approximately 20 ns.

Figure 40. DTCx Pins Tied to GND–Adaptive Dead Time Control

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