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LT7101

Rev. 0

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APPLICATIONS INFORMATION

A phase-locked loop (PLL) is also available on the LT7101

to synchronize the internal oscillator to an external clock

source that is connected to the PLLIN/MODE pin. Once

synchronized, the turn-on of the top MOSFET is aligned

to the rising edge of the synchronizing signal. See Phase-

Locked Loop and Frequency Synchronization section

for details.

Setting the Light-Load Operating Mode

The LT7101 can be set to enter high efficiency Burst Mode

operation, constant frequency pulse-skipping mode or

forced continuous conduction mode at light load currents.

To select Burst Mode operation, tie the PLLIN/MODE pin

to ground. To select forced continuous operation, tie the

PLLIN/MODE pin to INTVCC. To select pulse-skipping

mode, tie the PLLIN/MODE pin to INTVCC through a 100k

resistor. When synchronized, the LT7101 operates in

forced continuous mode. Table 2 summarizes the use of

the PLLIN/MODE pin to select light-load operating mode.

Table 2. Mode Selection

PLLIN/MODE PIN

LIGHT-LOAD OPERATING MODE

SGND

Burst Mode Operation

INTVCC

Forced Continuous Mode

R = 100k to INTVCC

Pulse-Skipping Mode

External Clock

Forced Continuous Mode

In general, the requirements of each application will dic-

tate 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 converter operates in discontinuous

operation. In addition, when the average output current

falls below approximately 100mA, the inductor current

will begin bursting at frequencies lower than the switching

frequency, and entering 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 reverse at light loads and switches at the same

frequency regardless of load. In this mode, the efficiency

at light loads is considerably lower than in Burst Mode

operation. However, continuous operation has the advan-

tage of lower output voltage ripple and less interference

to audio circuitry. in forced continuous mode, the output

ripple is independent or load current.

In pulse-skipping mode, constant frequency operation

is maintained down to approximately 1% of designed

maximum output current. At very light loads, the PWM

comparator may remain tripped for several cycles and

force the top MOSFET to stay off for the same number of

cycles (i.e., skipping pulses). The inductor current is not

allowed to reverse (discontinuous operation). This mode,

like forced continuous operation, exhibits low output rip-

ple as well as low audio noise and reduced RF interference

as compared to Burst Mode operation. It provides higher

low current efficiency than forced continuous mode, but

not nearly as high as Burst Mode operation. Consequently,

pulse-skipping mode represents a compromise between

light load efficiency, output ripple and EMI.

In some applications, it may be desirable to change light-

load operating mode based on the conditions present in

the system. For example, if a system is inactive, one might

select high efficiency Burst Mode operation by keeping the

PLLIN/MODE pin set to 0V. When the system wakes, one

might send an external clock to PLLIN/MODE to switch to

forced continuous mode. Such on-the-fly mode changes

can allow an individual application to benefit from the

advantages of each light-load operating mode.

Inductor Value Selection

For a given input and output voltage, the inductor value

and operating frequency determine the inductor ripple

current. More specifically, the inductor ripple current

decreases with higher inductor value or higher operating

frequency according to the following equation:

∆IL =

VOUT

f •L

⎛

⎝⎜

⎞

⎠⎟

1–

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

For proper operation, always use an inductor value that

is greater than:

LMIN > 520nH • VOUT

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