LTC3586 datasheet(23/36 Pages) LINER

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LTC3586/LTC3586-1

3586fb

always on (and switch B always off), to step-up the input

voltage to the programmed output. If VIN3 is close to the

programmed VOUT3, then the converter will operate in

4-switchmode.Inthiscasetheswitchessequencethrough

the pattern of AD, AC, BD to either step the input voltage

up or down to the programmed output.

Buck-Boost Regulator Burst-Mode Operation

In Burst Mode operation, the buck-boost regulator uses

a hysteretic FB3 voltage algorithm to control the output

voltage. By limiting FET switching and using a hysteretic

control loop, switching losses are greatly reduced. In this

mode output current is limited to 50mA typical. While

operating in Burst Mode operation, the output capacitor

is charged to a voltage slightly higher than the regulation

point. The buck-boost converter then goes into a sleep

state, during which the output capacitor provides the load

current. The output capacitor is charged by charging the

inductor until the input current reaches 250mA typical

and then discharging the inductor until the reverse current

reaches 0mA typical. This process is repeated until the

feedback voltage has charged to 6mV above the regulation

point. In the sleep state, most of the regulator’s circuitry

is powered down, helping to conserve battery power.

When the feedback voltage drops 6mV below the regula-

tion point, the switching regulator circuitry is powered on

and another burst cycle begins. The duration for which

the regulator sleeps depends on the load current and

output capacitor value. The sleep time decreases as the

load current increases. The buck-boost regulator will not

go to sleep if the current is greater than 50mA, and if the

load current increases beyond this point while in Burst

Mode operation the output will lose regulation. Burst

Mode operation provides a signiﬁcant improvement in

efﬁciencyatlightloadsattheexpenseofhigheroutputripple

when compared to PWM mode. For many noise-sensitive

systems, Burst Mode operation might be undesirable at

certain times (i.e. during a transmit or receive cycle of a

wireless device), but highly desirable at others (i.e. when

the device is in low power standby mode). The MODE pin

is used to enable or disable Burst Mode operation at any

time, offering both low noise and low power operation

when they are needed.

Buck-Boost Regulator Soft-Start Operation

Soft-start is accomplished by gradually increasing the

maximum VC3 voltage over a 0.5ms (typical) period.

Ramping the VC3 voltage limits the duty cycle and thus

the VOUT3 voltage minimizing output overshoot during

startup.Asoft-startcycleoccurswheneverthebuck-boost

is enabled, or after a fault condition has occurred (thermal

shutdown or UVLO). A soft-start cycle is not triggered by

changing operating modes. This allows seamless output

operation when transitioning between Burst Mode opera-

tion and PWM mode.

SYNCHRONOUS BOOST DC/DC SWITCHING

REGULATOR

The LTC3586/LTC3586-1 contain a 2.25MHz constant-

frequency current mode synchronous boost switching

regulatorwithtrueoutputdisconnectfeature.Theregulator

provides at least 800mA of output load current and the

output voltage can be programmed up to a maximum of

5V. The converter is enabled by pulling EN4 high. The

boost regulator also includes soft-start to limit inrush

current and voltage overshoot when powering on, short

circuit current protection and switch node slew limiting

circuitry for reduced radiated EMI.

Error Amp

The boost output voltage is programmed by a user-sup-

plied resistive divider returned to the FB4 pin. An internally

compensated error ampliﬁer compares the divided output

voltage with an internal 0.8V reference and adjusts the

voltage accordingly until FB4 servos to 0.8V.

Current Limit

Lossless current sensing converts the NMOS switch cur-

rent signal to a voltage to be summed with the internal

slope compensation signal. The summed signal is then

compared to the error ampliﬁer output to provide a peak

current control command for the peak comparator. Peak

switch current is limited to 2.4A independent of output

voltage.

operaTion

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