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LTC3589/LTC3589-1/

LTC3589-2

3589fe

Operating Modes

Table 10 shows the I2C command registers used to control

the operating modes of the LTC3589 buck-boost converter.

When command register SCR1 bit 6 is LOW, the LTC3589

buck-boost switching regulator operates in a ﬁxed fre-

quency pulse width modulation mode using voltage mode

feedback control. A proprietary switching algorithm allows

the converter to transition between buck, buck-boost, and

boost modes without discontinuity in inductor current or

loop characteristics. The switch topology is shown in the

application circuit in Figure 4.

When the input voltage is significantly greater than the

output voltage, the buck-boost converter operates in

buck mode. Switch D turns on continuously and switch C

remains off. Switches A and B are pulse width modulated

to produce the required duty cycle to support the output

regulation voltage. As the input voltage decreases, switch A

remains on for a larger portion of the switching cycle.

When the duty cycle reaches approximately 85%, the

switch pair AC begins turning on for a small fraction of the

switching period. As the input voltage decreases further,

the AC switch pair remains on for longer durations and

the duration of the BD phase decreases proportionately.

As the input voltage drops below the output voltage, the

AC phase will eventually increase to the point that there is

no longer any BD phase. At this point, switch A remains

on continuously while switches CD operate as a boost

converter to regulate the desired output voltage.

The buck-boost is set to Burst Mode operation by writing

a 1 to command register SCR1 bit 6. Using Burst Mode

operation at light loads improves efficiency and reduces

standby current at zero loads. In Burst Mode operation,

the inductor is charged with bursts of fixed peak amplitude

current pulses. The current pulses are repeated as often

as necessary to maintain the target output voltage. The

maximum output current that can be supplied in Burst Mode

operation is dependent upon the input and output voltage.

Typically IOUT(MAX) in Burst Mode operation is equal to:

IOUT(MAX) =

0.28 • VIN

VOUT+ VIN

(A)

OPERATION

If the buck-boost load exceeds the maximum Burst Mode

current capability then the output rail will lose regula-

tion and the power good comparator will indicate a fault

condition.

When the LTC3589 buck-boost is not enabled, a 2.5k pull-

down resistor is connected between BB_OUT and ground.

Soft-Start

The buck-boost converter has an internal voltage mode soft-

start circuit that ramps the buck-boosts error amp reference

from 0V to 800mV at a rate of 2V/ms. During soft-start,

the converter is regulating to the ramping reference and

will respond to output load transients. During soft-start the

buck-boost converter is forced into continuous mode opera-

tion regardless of the state of the SCR1 command register.

Current Limit Operation

The LTC3589 buck-boost regulator has current limit circuits

to limit forward current through the A switch and reverse

current through the D switch. The primary forward cur-

rent limit circuit injects a small fraction of the inductor

current into the feedback node whenever the inductor

current exceeds 2.7A (typical). Forcing the current into

the feedback node in the high gain feedback circuit has

the effect of lowering the output voltage until the aver-

age current in switch A is equal to the current limit. The

average limit uses the error amplifier in its active linear

state so once the fault condition is removed the recovery

is smooth with little overshoot.

A hard short on the output of the buck-boost will cause the

inductor current to exceed the 2.7A average current limit.

A second current limit turns off switch A in the event peak

inductor current reaches 3A (typical). The instantaneous

forward current limit provides extra protection in the event

of a sudden hard short.

The reverse current comparator on the D switch moni-

tors the current entering the BB_OUT pin. When this

current exceeds 1A (typical) switch D will turn off for the

remainder of the switching cycle. This feature protects

the buck-boost converter from excessive reverse current

if the buck-boost output is held above the regulation point

by an external source.

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