LTC3586EUFE#PBF Datasheet LTC3586EUFE#PBF, LTC3586EUFE-1#PBF, LTC3586EUFE-1#TRPBF | P22-P24

LTC3586/LTC3586-1

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Buck Regulator Dropout Operation

It is possible for a buck regulator’s input voltage, V

INx

, to

approach its programmed output voltage (e.g., a battery

voltage of 3.4V with a programmed output voltage of 3.3V).

When this happens, the PMOS switch duty cycle increases

until it is turned on continuously at 100%. In this dropout

condition, the respective output voltage equals the buck

regulator’s input voltage minus the voltage drops across

the internal P-channel MOSFET and the inductor.

Buck Regulator Soft-Start Operation

Soft-start is accomplished by gradually increasing the

peak inductor current for each buck regulator over a 500µs

period. This allows each output to rise slowly, helping

minimize the battery in-rush current. A soft-start cycle

occurs whenever a given buck regulator 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 modes.

Buck Regulator Switching Slew Rate Control

The buck regulators contain new patent pending circuitry

to limit the slew rate of the switch node (SW1 and SW2).

This new circuitry is designed to transition the switch node

over a period of a couple of nanoseconds, signiﬁcantly

reducing radiated EMI and conducted supply noise.

UCK-BOOST DC/DC SWITCHING

REGULATOR

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

frequency voltage-mode buck-boost switching regulator.

The regulator provides up to 1A of output load current.

The buck-boost can be programmed to a minimum output

voltage of 2.5V and can be used to power a microcon-

troller core, microcontroller I/O, memory, disk drive, or

other logic circuitry. The converter is enabled by pulling

EN3 high. To suit a variety of applications, a selectable

mode function allows the user to trade-off noise for ef-

ﬁ

ciency. Two modes

are available to control the operation

of the LTC3586/LTC3586-1’s buck-boost regulator. At

moderate to heavy loads, the constant frequency PWM

mode provides the least noise switching solution. At

lighter loads Burst Mode operation may be selected. The

output voltage is programmed by a user-supplied resistive

divider

returned to FB3. An error

ampliﬁer compares the

divided output voltage with a reference and adjusts the

compensation voltage accordingly until the FB3 pin has

stabilized to the reference voltage (0.8V). The buck-boost

regulator includes a soft-start to limit inrush current and

voltage overshoot when powering on, short-circuit cur-

rent protection, and switch node slew limiting circuitry

for reduced radiated EMI.

Input Current Limit

The input current limit comparator will shut the input

PMOS switch off once current exceeds 2.5A (typical). The

2.5A input current limit also protects against a grounded

OUT3

node.

Output Overvoltage Protection

If the FB3 node were inadvertently shorted to ground, then

the

output would increase indeﬁ

nitely with the maximum

current that could be sourced from V

IN3

. The LTC3586/

LTC3586-1 protect against this by shutting off the input

PMOS if the output voltage exceeds 5.6V (typical).

Low Output Voltage Operation

When the output voltage is below 2.65V (typical) during

start-up, Burst Mode operation is disabled and switch D

is turned off (allowing forward current through the well

diode and limiting reverse current to 0mA).

Buck-Boost Regulator PWM Operating Mode

In PWM mode the voltage seen at FB3 is compared to the

reference voltage (0.8V). From the FB3 voltage an error

ampli

ﬁer generates an error

signal seen at V

. This error

signal commands PWM waveforms that modulate switches

A, B, C, and D. Switches A and B operate synchronously

as do switches C and D. If V

IN3

is signiﬁcantly greater

than the programmed V

OUT3

, then the converter will op-

erate in buck mode. In this case switches A and B will be

modulated, with switch D always on (and switch C always

off), to step-down the input voltage to the programmed

output. If V

IN3

is signiﬁcantly less than the programmed

OUT3

, then the converter will operate in boost mode. In

this case switches C and D are modulated, with switch A

operaTion

LTC3586/LTC3586-1

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

voltage to the programmed output. If V

IN3

is close to the

programmed V

OUT3

, then the converter will operate in

4-switch mode. In this case the switches sequence through

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 signiﬁ

cant improvement in

efﬁcienc

y at light loads at the expense of higher output ripple

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 V

voltage over a 0.5ms (typical) period.

Ramping the V

voltage limits the duty cycle and thus

the V

OUT3

voltage minimizing output overshoot during

startup. A soft-start cycle occurs whenever the buck-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.

YNCHRONOUS BOOST DC/DC SWITCHING

REGULATOR

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

frequency current mode synchronous boost switching

regulator with true output disconnect feature. The regulator

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 ampliﬁ

er 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

LTC3586/LTC3586-1

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Zero Current Comparator

The zero current comparator monitors the inductor current

to the output and shuts off

the synchronous rectiﬁer once

the current drops to approximately 65mA. This prevents

the inductor current from reversing in polarity thereby

improving efﬁciency at light loads.

Antiringing Control

The antiringing control circuitry prevents high frequency

ringing of the SW pin as the inductor current goes to zero

in discontinuous mode. The damping of the resonant

circuit formed by L and C

(capacitance of the SW4

pin) is achieved internally by switching a 150Ω resistor

across the inductor.

PMOS Synchronous Rectiﬁer

To prevent the inductor current from running away,

the

PMOS synchronous rectiﬁer

is only enabled when

OUT

> (V

+ 130mV).

Output Disconnect and Inrush Limiting

The LTC3586/LTC3586-1 boost converter is designed to

allow true output disconnect by eliminating body diode

onduction of the internal PMOS rectiﬁer. This allows V

OUT

to go to zero volts during shutdown, drawing zero current

from the input source. It also allows for inrush current

limiting at start-up, minimizing surge currents seen by the

input supply. Note that to obtain the advantage of output

disconnect, there must not be an external Schottky diode

connected between the SW4 and V

OUT4

pin.

Short Circuit Protection

Unlike most boost converters, the LTC3586/LTC3586-1

boost converter allows its output to be short-circuited

due to the output disconnect feature. It incorporates

internal features such as current limit foldback and thermal

shutdown for protection from an excessive overload or

short circuit.

> V

OUT

Operation

The LTC3586/LTC3586-1 boost converter will maintain

voltage regulation even if the input voltage is above

the output voltage. This is achieved by terminating the

switching of the synchronous PMOS and applying V

IN4

statically on its gate. This ensures that the slope of the

inductor current will reverse during the time when cur-

rent is ﬂowing to the output. Since the PMOS no longer

acts as a low impedance switch in this mode, there will

be more power dissipation within the IC. This will cause

sharp drop in the ef

ﬁciency (see Typical Performance

Characteristics, Boost Efﬁciency vs V

IN4

). The maximum

output current should be limited in order to maintain an

acceptable junction temperature.

Boost Soft-Start

The LTC3586/LTC3586-1 boost converter provides soft-

start by slowly ramping the peak inductor current from

zero to a maximum of 2.4A in about 500µs. Ramping the

peak inductor current limits transient inrush currents

during start-up. A soft-start cycle occurs whenever the

boost is enabled, or after a fault condition has occurred

(thermal shutdown or UVLO).

Boost Overvoltage Protection

If the FB4 node were inadvertently shorted to ground, then

he boost converter output would increase indeﬁnitely with

the maximum current that could be sourced from V

IN4

. The

LTC3586/LTC3586-1 protects against this by shutting off

the main switch if the output voltage exceeds 5.3V.

operaTion

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LTC3586EUFE#PBF

Mfr. #:

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Manufacturer:

Analog Devices / Linear Technology

Description:

Battery Management High Efficiency USB Power Manager + Dual Buck + Boost + Buck/Boost DC/DC

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LTC3586EUFE#PBF

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