LTC3100 datasheet(14/24 Pages) LINER

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LTC3100

3100fa

buck converters. In this case, both converters will remain in

ﬁxed frequency operation, even at light load currents. If the

load is very light, they will exhibit pulse-skip operation.

If MODE is raised above 0.9V, or left open, Burst Mode

operation will be enabled for both converters. In this case,

either converter may enter Burst Mode operation at light

load, and return to ﬁxed frequency operation when the

load current increases. Refer to the Typical Performance

Characteristics section to see the output load Burst Mode

threshold vs VIN and VOUT. The two converters can enter or

leave Burst Mode operation independent of each other.

In Burst Mode operation, each converter still switches at

a frequency of 1.5MHz, using the same error ampliﬁer

and loop compensation for peak current mode control.

This control method eliminates any output transient

when switching between modes. In Burst Mode opera-

tion, energy is delivered to the output until it reaches the

nominal regulation value, then the LTC3100 transitions to

sleep mode where the outputs are off and the LTC3100

consumes only 15μA of quiescent current from VBST. Once

the output voltage has drooped slightly, switching resumes

again. This maximizes efﬁciency at very light loads by

minimizing switching and quiescent losses. Burst Mode

operation output ripple is typically 1% peak-to-peak.

Burst Mode operation for the boost converter is inhibited

during start-up, and until soft-start is complete and VBST

is at least 0.24V greater than VINBST.

Short-Circuit Protection

The LTC3100 output disconnect feature allows output

short-circuit while maintaining a maximum internally set

current limit. To reduce power dissipation under short-

circuit conditions, the boost peak switch current limit is

reduced to 400mA (typical).

Schottky Diode

Although it is not required, adding a Schottky diode from

SWBST to VBST will improve efﬁciency by about 2%. Note

that this defeats the boost output disconnect and short-

circuit protection features.

BUCK CONVERTER OPERATION

The buck converter provides a high efﬁciency, lower voltage

output and supports 100% duty cycle operation to extend

battery life. The buck converter uses the same 1.5MHz

oscillator used by the boost converter.

PWM Mode Operation

When the MODE pin is held low, the LTC3100 buck converter

uses a constant-frequency, current mode control architec-

ture. Both the main (P-channel MOSFET) and synchronous

rectiﬁer (N-channel MOSFET) switches are internal. At

the start of each oscillator cycle, the P-channel switch

is turned on and remains on until the current waveform

with superimposed slope compensation ramp exceeds the

error ampliﬁer output. At this point, the synchronous

rectiﬁer is turned on and remains on until the inductor

current falls to zero or a new switching cycle is initiated.

As a result, the buck converter operates with discontinuous

inductor current at light loads which improves efﬁciency.

At extremely light loads, the minimum on-time of the main

switch will be reached and the buck converter will begin

turning off for multiple cycles (pulse-skipping) in order

to maintain regulation.

Burst Mode Operation

When the MODE pin is forced high, or left open, the buck

converter will automatically transition between Burst Mode

operation at sufﬁciently light loads (below approximately

10mA) and PWM mode at heavier loads. Burst Mode opera-

tion entry is determined by the peak inductor current and

therefore the load current at which Burst Mode operation

will be entered depends on the input voltage, the output

voltage and the inductor value. Typical curves for Burst

Mode operation entry threshold are provided in the Typical

Performance Characteristics section of this data sheet. The

quiescent current on VINBK in Burst Mode operation is only

15μA. If the boost converter is enabled and VINBST or VBST

are at a higher potential than VINBK, some of the quiescent

current will be supplied by the boost converter, reducing

the burst quiescent current on VINBK to just 9μA.

OPERATION

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