LTC3129EMSE datasheet(12/30 Pages) LINER

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LTC3129

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inductor current is approximately 275mA (typical). In

Buck mode, the output current is approximately equal to

the inductor current IL.

IOUT(BUCK) ≈ IL • 0.89

The 90ns SW1/SW2 forced low time on each switching

cycle briefly disconnects the inductor from VOUT and VIN

resulting in about 11% less output current in either buck

or boost mode for a given inductor current. In boost mode,

the output current is related to average inductor current

and duty cycle by:

IOUT(BOOST) ≈ IL • (1 – D) • Efficiency,

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

givenoutputcurrentcomparedtobuckmode,theefficiency

in boost mode will be lower due to higher IL² • RDS(ON)

losses in the power switches. This will further reduce the

outputcurrentcapabilityinboostmode.Ineitheroperating

mode, however, the inductor peak-to-peak ripple current

does not play a major role in determining the output cur-

rent capability, unlike peak current mode control.

With peak current mode control, the maximum output

current capability is reduced by the magnitude of inductor

ripplecurrentbecausethepeakinductorcurrentlevelisthe

control variable, but the average inductor current is what

determines the output current. The LTC3129 measures

and controls average inductor current, and therefore, the

inductor ripple current magnitude has little effect on the

maximum current capability in contrast to an equivalent

peak current mode converter. Under most conditions in

buck mode, the LTC3129 is capable of providing a mini-

mum of 200mA to the load. In boost mode, as described

previously, the output current capability is related to the

boost ratio or duty cycle (D). For example, for a 3.6V VIN

to 5V output application, the LTC3129 can provide up

to 150mA to the load. Refer to the Typical Performance

characteristics section for more detail on output current

capability.

operaTion

Overload Current Limit and IZERO Comparator

The internal current sense waveform is also used by the

peakoverloadcurrent(IPEAK)andzerocurrent(IZERO)com-

parators. The IPEAK current comparator monitors Isense

and turns off switch A if the inductor current level exceeds

its maximum internal threshold, which is approximately

500mA. An inductor current level of this magnitude will

occur during a fault, such as an output short-circuit, or

during large load or input voltage transients.

TheLTC3129featuresneardiscontinuousinductorcurrent

operation at light output loads by virtue of the IZERO com-

parator circuit. By limiting the reverse current magnitude

in PWM mode, a balance between low noise operation and

improved efficiency at light loads is achieved. The IZERO

comparator threshold is set near the zero current level in

PWMmode,andasaresult,thereversecurrentmagnitude

will be a function of inductance value and output voltage

due to the comparator's propagation delay. In general,

higher output voltages and lower inductor values will

result in increased reverse current magnitude.

In automatic Burst Mode operation (PWM pin low), the

IZERO comparator threshold is increased so that reverse

inductor current does not normally occur. This maximizes

efficiency at very light loads.

Burst Mode OPERATION

When the PWM pin is held low, the LTC3129 is config-

ured for automatic Burst Mode operation. As a result,

the buck-boost DC/DC converter will operate with normal

continuous PWM switching above a predetermined mini-

mumoutputloadandwillautomaticallytransitiontopower

saving Burst Mode operation below this output load level.

Note that if the PWM pin is low, reverse inductor current is

not allowed at any load. Refer to the Typical Performance

Characteristics section to determine the Burst Mode

transition threshold for various combinations of VIN and

VOUT. If PWM is low, at light output loads, the LTC3129

will go into a standby or sleep state when the output volt-

age achieves its nominal regulation level. The sleep state

halts PWM switching and powers down all non-essential

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