LT3154AVPBF datasheet(16/32 Pages) AD

16 / 32 page

LT3154

Rev. 0

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OPERATION

In buck mode, the output current is approximately equal

to the inductor current IL:

IOUT(BUCK)≈ IL

In boost mode, the output current is related to average

inductor current and duty cycle by:

IOUT(BOOST)≈ IL • (1‑D)

where D is the converter duty cycle

Since the output current in boost mode is reduced by the

duty cycle (D), the output current rating in buck mode

is always greater than in boost mode. Also, because

boost mode operation requires a higher inductor current

for a given output current compared to buck mode, the

efficiency in boost mode will be lower due to higher con‑

duction (IL² × RDS(ON)) losses in the power switches. This

will further reduce the output current capability in boost

mode. In either operating mode, however, the inductor

peak‑to‑peak ripple current does not play a major role

in determining the output current capability, unlike peak

current mode control.

The maximum output current capability in PWM mode

curves in the Typical Performance Characteristics section

show the relationship of input voltage and the ability to

deliver load current at VOUT = 3.3V. If VOUT drops below

approximately 1V, the inductor current will be reduced

by approximately 50% to limit power dissipation during

a short circuit.

Peak, Reverse and Zero Current Comparators

Theinternalcurrentsensewaveformsareusedbythe Peak

(IPEAK), Reverse (IREVERSE) and Zero current (IZERO) com‑

parators. The IPEAK current comparator monitors ISENSE

and interrupts normal PWM operation if the inductor

current level exceeds its maximum internal threshold.

This threshold is approximately 33% above the maxi‑

mum average current level of the current control loop or

9.5 Amps. If the internal current sense waveform rises

above this level, the LT3154 will disconnect the inductor

from VIN by shutting off switch A to prevent higher current

in the inductor. The IPEAK circuitry is reset by the oscillator

clock at the end of each switching cycle. In the event that

the IPEAK comparator is tripped as the result of an output

short circuit condition, where VOUT is discharged below

approximately 1V, the LT3154 will initiate a soft‑start

event keeping the on‑chip power dissipation to low levels.

Once the short circuit is removed, the LT3154 will restart

in the normal fashion. If the average current loop is able

to prevent inductor current from reaching IPEAK during a

short circuit event, soft‑start will not be initiated, but the

maximum current capability of the current loop will be

reduced by 50% to reduce power dissipation.

In addition to controlling the maximum inductor current,

the LT3154 contains sense circuitry on the D switch to

limit the reverse current as well. The amount of reverse

current allowed depends on the mode of operation. In

PWM mode (SYNC/MODE = 1), the typical reverse cur‑

rent limit is –1.2A, providing clean, fixed frequency switch

operation at light loads and during load step transients.

In BURST mode (SYNC/MODE = 0), the LT3154 operates

withatraditionalzerocurrentcomparator,maintaininghigh

efficiency at light loads and during burst packet intervals.

Once IREVERSE or IZERO is detected, the D switch is shut

off for the remainder of the switching cycle and reset by

the oscillator clock at the end of the cycle.

Oscillator

The frequency of operation is programmed by an external

resistor from the RT pin to ground, according to the fol‑

lowing equation:

RT(kΩ)=

110

fSW MHz

(

)

Therecommendedfrequencyrangeisbetween400kHzand

4MHz, allowing efficiency and external component sizes

to be optimized for each application. An internal 2.2MHz

oscillator frequency can be selected by connecting RT to

VIN, eliminating the need for an external RT resistor. The

LT3154 can also be synchronized to an external clock

source using the SYNC/MODE pin and an internal phase

lock loop (PLL). If synchronization to an external source

is desired, the value of the RT resistor should be chosen

at a frequency 25% to 50% below the applied clock signal

for proper operation.

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